Many factors in outdoor environment，such as light，shadows and trees blocks etc.will affect the video unusual event detection based on the hidden Markov model(HMM).So using the independent component analysis (ICA) and HP (Hodrick-Prescott) filter，a new HMM video unusual event detection method is proposed.This method first employed ICA to construct a normal video feature subspace，and projected the image sequence into this subspace to achieve data reduction.Then the trend component in feature sequence caused by the environment factors is cancelled by the HP filter.So to overcome the adverse environmental factors，the effect of the proposed method is improved.Using the video data in a forbidden road for motor vehicles，the test result shows the proposed method can detect the video unusual events effectively in the complex outdoor environments.