The invention discloses a method for synthesizing 
femto-second grade ultrashort pulses based on cerenkov 
radiation. The method includes the steps that 1, a reference light path is established, non-linear crystals are put on a six-dimension rotary horizontal-movement platform, the angle of the non-linear crystals is adjusted by rotating the six-dimension rotary horizontal-movement platform, and therefore non-linear cerenkov 
radiation occurs on the inner surfaces of the non-linear crystals; 2, according to the reflecting angle of incident 
laser on the inner surfaces of the non-linear crystals, the non-linear cerenkov 
radiation angle of each level of frequency doubling light in the crystals is determined; 3, 
fundamental frequency light, 
double frequency light, 
triple frequency light, quadruplicated frequency light, quintuple frequency light and higher-order frequency-doubling light generated by the incident 
laser emerge at own radiation angles, and are modulated by an amplitude type 
liquid crystal light modulator and a phase type 
liquid crystal light modulator, parallel transmission directions of all orders of light 
waves are changed into converging transmission directions through a second dispersing 
prism, and the light 
waves are synthesized into one beam of 
femto-second grade ultrashort pulses through a third dispersing 
prism. With the method, the synthesizing process of 
femto-second grade 
light wave is achieved, efficiency is high, and fidelity is high.