Abstract: In favor of seamless connected with main program of pressure simulation model of trapped oil, analytical calculation method of relief area was necessary. In a trapped periods, based on the trapped oil principle and the sweeping-area method, firstly derived the precise relation of relief slot and gear transmission; secondly achieved the precise results of dynamic relief area and dynamic trapped area followed turn angle, and for this precise, an actual transition curve machined by generating method was substituted for traditional circle curve in tooth profile. Based on an instance, final simulation results was consistent with present literatures, which validated the calculation model of relief area was all right, and indicated their variation curve in a trapped period was the same as a high-order parabola. Educed the precise calculation of relief area and trapped area was a powerful pledge for the reliability of pressure simulation model, and the precise calculation method on relief area was universal for any style relief slot.