By using the ion beam assisted deposition to evaporate the materials and utilizing the double film thickness monitoring method to control the thickness of each layer, a depolarised dichroic mirror with the incidence angle of 45°, the spectrum 808nm reflected and 1064nm transmitted was prepared, which was used in an all solid state 355 nm laser. The partial thin film samples were annealed at 250℃, then the transmittance was measured with a Lambda 950 spectrophotometer; the weak absorption value of coatings was measured by a surface thermal lensing technique before and after annealing; the laser-induced damage threshold was tested by a 1 064 nm Q-switch pulsed laser; the morphology of the samples was observed by NIKON microscope under the different laser energy. The experimental results show that, the transmittances of the films are T1064 is 99.6%, T808 is 0.04%, which meet the design requirements, and the optical performance of the film can meet the requirements of the all solid state 355nm ultraviolet laser system; the weak absorption value is decreased after annealing; Micro destruction of the film is emitted under the action of laser, which proves that the film will not evolve to a catastrophic failure.