Failure of slope is progressive process. The shear strength parameters c、? have different exerting extents during the destroy process, so the strength reserve is different. Thus, the determining of double safety factors based on the strength reserve and the slope stability evaluation based on double safety factors are worthy of investigation. On the basis of softening regularities of geo-material and assumption of linear distribution of strength parameters, the correlation between the reduction factor of cohesion and the reduction factor of inner friction angle is deduced in this article. Then the correlation is introduced into Bishop’s slice method, the non-proportional double safety factors Bishop’s slice method is developed. The slope stability can be evaluated with comprehensive safety factor based on weight of strength parameters contributing to the resistant shear force. This method not only overcomes the blindness of the existing double safety factors method to determine the double safety factors but also provides a new approach to applying the double safety factors method to engineering practice. Finally, the proposed and traditional methods are both applied in an example and the resemblance of their output validates the suggested method.