Constant false alarm rate of the radar signal is an indispensable content in the auto examination. There are many methods about accounting the examining limit. Because of the distinct methods which have the different disadvantages and limits, they can’t satisfy with the integrated request by using the constant false alarm. Rayleigh fitting which is a new disposal method of constant false alarm is presented in this paper. Compared with the method of traditional constant false alarm, this method has the remarkable advantages in multi-target examining, and it can satisfy the guideline request of radar easier in examining the limit precision.