为改进噪信比的计算方法，利用系统相关函数的Ｚ变换和逆Ｚ变换关系，导出了系统噪信比的计算公式，它使得噪信比可以直接通过系统模型和噪声模型传递函数的复积分得到。同时，给出了这个复积分的递推计算式，避免了复积分留数的复杂运算，简化了用数字仿真确定噪信比的计算量。并将这一方法推广用于计算多变量系统的噪信比。数值例子说明给出方法是有效的。 In order to improve the calculation method of noise ratio, the calculation formula of system noise ratio is deduced by using the relationship between Z transform and inverse Z transform of system correlation function, which makes the noise ratio can directly transfer the complex integral of system function and noise model get. At the same time, the recursive formula of the complex integral is given, which avoids the complicated operation of the complex integrals and simplifies the calculation of the noise ratio by digital simulation. This method is generalized to calculate the noise ratio of multivariable systems. The numerical example shows that the method is effective.