In order to improve the adaptability of tomato seedling leaves to drought stress，and enhance the drought resistance of tomato，the changes of tomato seedling growth and protective enzyme activity after foliar spraying of chitooligosaccharide(COS) were studied，which can provid reference for drought resistance of tomato.Taking tomato seedlings as test materials，a pot experiment was conducted in College of Resources and Environment，Jilin Agricultural University.Tomato seedlings were treated with COS and drought，and 6 treatments were set up:1.non-drought + clear water，control；2.non-drought + COS 50 mg/L；3.non-drought + COS 100 mg/L；4.drought + clear water；5.drought + COS 50 mg/L；6.drought + COS 100 mg/L.After spraying COS，the plant height，stem diameter and SPAD value of tomato seedlings were determined after the soil relative water content reached the drought stress condition (the soil water content was 40%～50% of the field water holding capacity) .The leaf protective enzyme activity，malondialdehyde(MDA)，proline and soluble protein content were determined on the 0，2，4 and 6 d after the soil water content reached the set drought level after spraying COS.The results showed that COS could significantly alleviate the decrease of plant height，stem diameter and SPAD value of tomato seedlings caused by drought，increase the activities of catalase (CAT)，peroxidase (POD) and superoxide dismutase (SOD) in tomato seedling leaves，increase the contents of proline，soluble protein and other osmotic substances.COS could alleviate the increase of MDA content，and the effect of high concentration (100 mg/L) COS was the best.In conclusion，100 mg/L COS could alleviate the damage of drought stress on tomato seedling growth.