With the peak-seeking servo-controlled technology based on a residual error,an external optical cavity can keep in resonance with the frequency of Ti-sapphire laser in the experiment. The servo loop uses a piezoelectric transducer (PZT) as a modulator,which modulates the intensity of the transmitted light passing through the cavity. The servo signal is obtained from the lock-in amplifier,then it is amplified by a high voltage amplifier,which adjusts the length of the cavity according to the laser frequency. In this approach,the resonant frequency of the cavity can be locked to the laser frequency for a long time as the laser operates in a fixed frequency and the relative fluctuation of output power from the cavity is lower than 2%. When the frequency of the laser scans,the cavity is also capable to follow the variation of laser frequency automatically in an uninterrupted range of 2 GHz.