Conventional market risk assessment methods mainly focus on the sellers' side， but ignore the inherent causes and economic principles. This cannot reflect the actual risks facing the electricity buyers and sellers. The paper proposes a stochastic model for the bidding behaviors of electricity buyers and sellers based on sequence operation theory. The market players bidding strategies and impacts on the bidding results are simulated. Risk assessment index is also derived from the utility function to comprehensively evaluate the risks from both the trading energy volume and prices. A case study in a deregulated electricity market validates the proposed method.