Atmospheric intensity scintillation effect deteriorates the communication quality of space chaos laser communication. To analyze the effect of internal mismatch and internal mismatch induced by intensity scintillation on bit error rate (BER) for space downlink chaos laser communication system, numerical simulation results based on BER formula are conducted and analyzed. Numerical results demonstrate that BER worsens with the effect of intensity scintillation. Although the probability that received optical power causes larger mismatch noises is small, its corresponding larger instantaneous BER deteriorates the ensemble average BER significantly. Results also indicate that internal mismatch deteriorates BER more than external mismatch. These results are beneficial for designing practical space chaos laser communication system with optimal performance.