The velocity of aerobat can be measured by laser Doppler velocimetry system and its accuracy is seriously affected by the attitude measurement errors. To precicely measure the three-dimensional (3D) velocity, an all-fiber triple-beam laser doppler velocimetry (TLDV) system based on coherent detection principle is proposed. Mathematical model of velocity measurement is described. According to the simulations, it is found that, firstly, effect of the attitude errors on the velocity measurement accuracy should not be neglected; secondly, with the pitch angle changing, influence of the pitch error and roll error on velocity measurement accuracy change; thirdly, the relationship between velocity accuracy and roll error is linear, while that between the accuracy and pitch error is nonlinear. The conclusions can be used for the design and velocity error correction of laser Doppler velocimetry system.