Market power monitoring is an important part of power market supervision and an indispensable means to realize the benign operation of the market. With the gradual liberalization of electricity market, the electricity retail market continues to expand, but the corresponding retail market monitoring mechanism is not sound. The paper analyzes the abuse of market power in the electricity retail market, designs a market power monitoring indicator system, clarifies the measurement standards of market power monitoring indicators, proposes a monitoring method of market power in the whole process of retail market transaction in three stages: before, during and after the event, and uses the grey comprehensive evaluation method to evaluate the market behavior of the electricity sales companies. Finally, by designing the examples of pre-prevention, in-process monitoring, and post-analysis, the effectiveness of the method in this paper is verified for the whole-process monitoring of the electricity retail market, which provides a reference for establishing the electricity retail market monitoring system in the future.