When imaging performance of optoelectronic systems to observe point targets or laser far-field effects is evaluated, Strehl ratio is usually used as evaluation criteria, and Strehl ratio is often applied in engineering applications. In order to understand the limits of application field and engineering feasibility, study on the approximate application limits is done. The three approximation ways are deduced, which can supply theoretical basis for the approximations. According to Strehl ratio theory, its accurate analysis method under actual aberration condition is obtained as well. The comparison is done between the approximations and accurate method from the view of imaging. Results show that when the wavefront phase error is smaller than 0.1 wavelength, the error between first approximation and second approximation with third approximation is smaller than 10%. When the obscuration ratio is 0.3 and relative error of Strehl ratio is less than 10%, the wavefront phase error variance must be less than 0.13 wavelength for the first approximation method, the wavefront phase error variance must be less than 0.1 wavelength for the second approximation method, and the wavefront phase error variance must be less than 0.17 wavelength for the third approximation method. The research can apply Strehl ration approximations under certain relative error conditions, which can provide technical reference for imaging performance or far-field laser effects assessment.