By combining femtosecond laser with the scanning near-field optical microscope (SNOM), we have successfully realized the femtosecond-resolved near-field optical system. Through high frequency acousto-optic modulation and difference frequency lock-in detection, the signal-to-noise ratio was greatly improved and the background signals of the pump and probe light were removed. Therefore, we can measure a femtosecond-resolved weak transmission change in the collection mode SNOM. Spatial resolution of 80nm and temporal resolution better than 200fs were achieved simultaneously. Using this system, we studied the relaxation processes of hot electrons in gold nanostrucures and the difference in relaxation dynamics was observed at different sample positions.