The invention relates to a cost evaluation method for low-temperature propellant long-term in-orbit evaporation capacity control. According to the method, Heat leakage quantities and exhaust available cooling capacities of various assemblies of a low-temperature propellant long-term in-orbit storage evaporation capacity control system are comprehensively considered, firstly, the evaporation capacity and evaporation rate of a low-temperature propellant long-term in-orbit storage system are acquired, weight cost caused by an evaporation capacity control measure is acquired, and then the optimal design working condition with the minimal weight cost is acquired and is used for guiding design of the low-temperature propellant long-term in-orbit storage system. System efficiency indexes such as low system weight, low power and implementability are adopted as optimizing targets, a project evaluation method is established, and analysis of the low-temperature propellant long-term in-orbit evaporation capacity control system can be effectively carried out.