Intense solar radiation leads to thermal deformation of the solar telescope opto-mechanical system, triggering obvious time-varying focus shifts. Defocus aberration has an adverse effect on high-resolution observation and reduces the image spatial resolution. Therefore, the defocus aberration is necessary to be detected and compensated. Considering the effects of atmospheric turbulence, the focus detection method based on general image processing can not be applied well on ground-based solar telescopes. The focus detecting method based on Shack-Hartman wavefront detection which commonly used in astronomical telescopes is invalid at observing the solar limb and the low contrast solar granulation. The focus detecting method based on image power spectrum analysis is provided. This method used the average value of the low-frequency components of the image spectrum ratio as the cost function for focus detection, which can effectively eliminate the impact of object structure and atmospheric turbulence. Experimental results show that the detecting accuracy and frequency of this method can meet the requirement of high-resolution observations for different objects on ground-based solar telescope.