Abstract: Taking a sphere and a short cylinder as instance and based on active sonar equation, numerical simulation means are used to study the plane wave impinge, steady scattering and far field conditions should be satisfied in experiments of measuring the target strength in this paper. The measuring error when just satisfying the basic far field condition and plane wave condition are discussed. Besides single "high light" echo, according to the forming mechanism of echo, puts forward that under the many "high-light" echo circumstances, the steady scattering condition is that the incident pulse width must be greater than the extended width of narrow pulse echo.