A scheme for measuring the intra-cavity round-trip loss of an all-solid-state single-frequency laser by inserting a type-I noncritical phase-matching nonlinear crystal introducing nonlinear loss into the resonator is presented. The intra-cavity round-trip loss is theoretically deduced by analyzing the dependence of the fundamental-wave (FW) and second-harmonic-wave (SHW) powers on the pump factor and the nonlinear conversion factor of the single-frequency laser and experimentally measuring them by recording different FW and SHW powers, which are decided by the temperature of the nonlinear crystal. The measured intra-cavity round-trip loss and pump factor are 4.84% and 6.91% W 1, respectively. The standard deviations of the measured intra-cavity round-trip loss and the pump factor are 0.26% and 0.07%, respectively. This scheme is very suitable for measuring the intra-cavity round-trip loss of a high-gain solid-state single-frequency laser.