The accuracy software safety level assessment can ensure the various software obtain the reasonable resource allocation. Most existing software safety level assessment approaches only consider the influence of the system hazards and don't consider the hazard occurrence probability level, the probability level that the system in the hazard conditions, and the other important occurrence factors. Thus these approaches can't assess the system safety level accurately. To solve this problem, a new software safety level comprehensive assessment approach is proposed in this paper based on the multidimensional system risk index. This method first determines four important influence factors of the software safety level according to the related standards, namely the hazard influence level ‘C’, the probability level ‘F’ that the system in the hazard condition, the probability level ‘P’ that system fails to control the hazard, and the hazard occurrence probability level ‘W’. Then, we propose the comprehensive assessment method of the system risk index with the four factors. Finally, the application results show that the proposed approach are more comprehensive and extensive, and has better practicability, effectiveness, and feasibility.