Multi-variant system uses diversified technology and dynamic redundancy strategy to achieve high security and high reliability of the system from the architecture level. However， the existing researches are rarely related to the quantitative evaluation of Quality of Service （QoS） for multi-variant system. For this problem， a QoS and loss evaluation method for multi-variant system was proposed. Firstly， on the basis of formal modeling of multi-variant system architecture， the QoS evaluation model and process were proposed based on QoS attribute and weight matrices， and the importances of QoS attributes were evaluated by information entropy method. Secondly， some typical examples of multi-variant system were constructed， and QoS attributes that affect the performance and security of the system were designed and selected. Finally， QoS attribute weight， QoS value， QoS difference and loss of the system were evaluated quantitatively， in which the average loss of performance attributes is 15.86 percentage points and the gain of safety attribute is 4.98 percentage points. Evaluation results show that the multi-variant mechanism has QoS loss， but its security attribute brings a certain degree of QoS gain， which provides a reference for constructing a multi-variant system with high QoS and low QoS loss.