Compared with other ancillary service markets， the characteristic of startup power supply（including blackstart generators） market is discussed. Resources which startup power supply service needs and the costs of participants are analyzed. From the viewpoint of risk management， a model of optimal purchase of startup power supply service is proposed， and optimal strategy and algorithm are designed. Both technical performances and economical indices of startup power supplies are considered comprehensively. Besides compensating the costs of service suppliers， the service fee also includes a market prompting signal which can lead service suppliers to improve their startup ability for recovery control. Issues such as market monitoring， the proportion and payment of recovery service fee are also mentioned. The validity of the model and algorithm is verified by simulations.