Third harmonic generation (THG) of Ti:sapphire 744nm laser is produced using two pieces of type Ⅰ BBO crystal while rotating the cardinal plane of the crystal. The relationship between the THG laser energy and the rotate angle is investigated in theory and experiment, the temporal and spatial characters of ω, 2ω, 3ω of output are detected. The seed source for KrF excimer MOPA laser system with good optical beam qualit y is obtained.