前置级不对称现象是导致射流管伺服阀零偏的主要因素。考虑两接收孔大小不相等、射流管与接收器不对中、接收器接收孔中心不对称等几何结构因素,建立了射流管伺服阀前置级的数学模型；针对接收器接收孔中心不对称的工况,建立了基于定积分的修正模型;考虑前置级接收面积的不对称度,提出了压力特性以及射流管伺服阀零偏值的定量分析方法。分析结果表明:前置级加工、装配和环境因素作用将造成几何结构的不对称现象,并直接造成伺服阀的零偏；射流管与接收器的初始装配误差和接收孔半径初始误差将严重导致伺服阀产生零偏；当右接收孔尺寸小于左接收孔、射流管存在向右偏移误差、右接收孔轴线与垂直方向夹角小于左接收孔与垂直方向夹角时,伺服阀产生正向零偏。文中还通过试验验证了理论的正确性，在伺服阀的加工和装配过程中应尽可能保证或创造条件做到前置级结构的对称。

The pre-stage asymmetry is the main factor causing the zero deviation of the jet-pipe servo valve. In view of the uneven size of two receiving holes, the misalignment of jet-pipe and the receiver and the asymmetry of the receiving hole center, a mathematical model of the jet-pipe pre-stage was established. A modified model based on definite integral was designed to deal with the asymmetry of the receiving hole center. Considering the asymmetry of the receiving area of the pre-stage, the quantitative analysis method and model of the pressure characteristics and the zero deviation value were established. The results show that the pre-stage processing, assembly and environmental effects will cause the asymmetry of the geometric structure and directly cause the zero deviation of the servo valve. The initial assembly error between the jet-pipe and the receiver, the initial error of the radius of the recei-ving hole will seriously cause the servo valve to produce a zero deviation. The servo valve produces a positive zero deviation, when the right receiving hole is smaller than the left receiving hole, the jet-pipe has an offset error to the right, and the angle between the axis of the right receiving hole and the vertical is smaller than the angle between the left receiving hole and the vertical. The experimental results are in accordance with the theoretical results. In the processing and assembly process of the servo valve, the symmetry of the pre-stage structure should be guaranteed as much as possible.