Electronic Commerce is a considerable wide spread business engineering model in the Internet. The analysis of the EC's security is a hot research topic in the present. The article studies the Secure Electronics Transactions (SET) protocol and uses the model checking tool SPIN to model and to verify the key part of the protocol-purchasing process protocol. We describe the abstract model of the protocol with promela language of SPIN. During the modeling process, we plant some exploits which the intruders can take advantage of. Then we use LTL to describe the attributes that the protocol should satisfy and carry out the model checking. In our experiment, the intruder is detected with the tool SPIN.