A, U-like shape on the curve of fundamental-mode waist in the active medium versus pump power, at different cavity lengths of the laser diode (LD) end-pumped Nd:YVO4 solid-state lasers with a flat-flat resonator structure, is analyzed by means of the transform circle approach. There are caves and pinnacles on the curve at appropriate cavity lengths. Utilizing the caves and pinnacles, a new method is put forward to determine the focal length of thermal tens. Based on the theoretical analysis, we have designed a laser and obtained some experimental results that are consistent with the theoretical expectations.