So far there are few reports about sediment and runoff on long-term location observation on watershed scale in karst area of southwestern China, and the related mechanism is not enough. Characteristics of runoff and sediment in the small watershed of karst area were analyzed based on the scale of small watershed, and the main influencing factors of sediment and runoff in karst watershed were discussed qualitatively and quantitatively. Huyangshui, a typical karst small watershed located in Zunyi, northern Guizhou province was selected as the study area, and the control station of small watershed, a field position observation method for small watershed scale was used in this study. At the control station, data of water level was recorded continuously by automatic gauge for converting to runoff yield, and the sediment concentration was measured by using drying method, which mainly includes the steps of sampling, filtering, drying and so on. The characteristics of runoff and sediment in this small watershed of different months and different years, especially the influences of rainfall on the characteristics of runoff and sediment were analyzed based on the continuous observation from 2010 to 2013. The results showed that from January to December, the monthly mean runoff and sediment yield increased first and then decreased gradually. However, the maximum of runoff and sediment yield in the small watershed of Huyangshui appeared in different months, this could be because of special erosion environmental factors in karst area such as the subterranean soil and water leakage and others. The peak of runoff occurred in October, the value of which was 639 000 m3, and the maximum of sediment yield appeared in June, the value of which was 116.21 t. Significance tests showed that runoff yield was no significant difference among months (P>0.05). However, the sediment yield in June was significantly higher than that in January, February, March and December (P<0.05), and among other months, sediment yield was no significant difference (P>0.05). Affected by the severe drought in Southwest China in 2010-2011, both of the runoff and sediment yield of Huyangshui small watershed in 2013 were significantly higher than that in other years from 2010 to 2012 (P<0.05). During the period of this study, the average sediment delivery modulus of Huyangshui small watershed was 215.32 t/(km2•a), that was close to the value of the soil erosion modulus in Guizhou karst area, 279.47 t/(km2•a), which was published by Guizhou government. The precipitation of the small watershed had an obvious effects on the characteristics of runoff and sediment. There was a significant correlation between rainfall and sediment and runoff (P<0.01), and a significant correlation between I60 and sediment and runoff(P<0.05). In addition to the content of above, the differences of soil loss between scales of slope and small watershed were discussed, generally speaking, the results which were observed at the scale of slope were smaller than that obtained from the scale of small watershed in karst area. The results of the study can provide modified reference for the application of the Universal Soil Loss Equation (USLE), Revised Universal Soil Loss Equation (RUSLE), Chinese Soil Loss Equation (CSLE), and so on in karst area. This study are useful for basic data accumulation for the establishment of soil loss equation which is suitable for karst area in the future.