In recent years, more than 90% of the signal laser power can be up-converted based on the high-efficiency double resonant external cavity sum-frequency generation (SFG), especially when the whole system runs under the undepleted pump approximation scheme. Therefore it is difficult to directly achieve an error signal with a high signal-to-noise ratio through the signal laser to lock its frequency to the cavity mode. In this paper a novel method, based on the frequency modulation of signal laser and demodulation of the SFG laser, is used to obtain the error signal to realize the cascade frequency locking between the two fundamental lasers and the external cavity. In this experiment, 1064 nm laser is the pump laser and 1583 nm laser is the signal laser. They are coupled into a ring cavity inside which a 5% MgO-doped PPLN (25 mm1 mm0.5 mm) is used to produce the SFG laser of 636 nm. When the pump laser is resonant with the external cavity, a circulating power of 14.3 W is obtained with its input power of 1.3 W. The reflectivity of the input coupling mirror of signal laser is 10% to restrain the impendence mismatch. The temperature of PPLN is set at 68.5 ℃ to reach the optimum SFG temperature. In order to keep the signal laser resonance inside the external cavity, one needs to lock its frequency to the cavity mode. A 28.5 kHz sinusoidal voltage is used to modulate the frequency of the signal laser so that the frequency of 636 nm laser is modulated simultaneously. Then 5% of the output 636 nm laser power is sent into a Si photodiode detector the signal of which is demodulated at the modulation frequency by a lock-in amplifier. Finally the demodulated signal is feedback to the frequency control port of signal laser. Under these conditions, 73% of 1583 nm signal laser power can be converted into 636 nm laser power when the incident power varies from 10 W to demodulation of the transmitted cavity mode of 1583 nm when the incident signal laser power is below 12 mW. When the signal laser power increases from 50 mW to 295 mW, the conversion efficiency linearly drops to 60%, which is mainly caused by depleting the 1064 nm pump laser power. Finally a 440 mW of 636 nm laser is generated with an incident signal laser power of 295 mW. This scheme can realize a high-efficiency SFG with a low input signal laser power or poor single-pass SFG efficiency.