It is important to calculate the biostatic scattering radar cross section of a thin perfectly conducting chaff to obtain the RCS of a mix-backfilled chaff cannonball. After establishing a reflection model of chaff, an integral equation was got for taking the current distribution of chaff. Then a numerical value was acquired to solve the equation by MOM. After that the expression of chaff radiation field was given. In the end, the calculating formula to compute biostatic scattering radar cross section was presented, and the simulating result of the average backscattering and biostatic radar cross section of chaff was shown.