This paper reports a simple and accurate method for measuring thermal focal length of LD-side-pumped laser crystal. While applying the nonsymmetrical plane-plane-cavity to measure the thermal focal length, the distance from the end of the laser crystal to the end mirror need not be zero, and the reliability of this method is proved by experiments. Based on the obtained measurement results, folding configuration oscillators of yellow laser for intracavity sum frequency can be optimized, which ensures high space overlapping of two fundamental beams and high sum frequency efficiency.