The invention provides a method for participating in electricity market clearing and safety checking by an unmodeled small unit. In an electric power spot market technical support system, when the physical model does not model the low-voltage-level small unit participating in the market, the active power transmitted to the main network by the small unit through the equivalent load is predicted byutilizing the equivalent load prediction active power through defining the hooking relationship and the power distribution coefficient of the small unit and the electrical bus or the equivalent load,and the real active power of the equivalent load is obtained to calculate the power flow. And the sensitivity values of the small unit and the main network branch are calculated by using the power distribution coefficient, the equivalent load and the sensitivity value of the main network branch, so that the influence of the output of the small unit can be considered in the DC power flow constraintin the optimization model construction. According to the method, the influence of unmodeled small unit power generation on the power grid tide and clearing result can be accurately considered in an optimized clearing and safety checking algorithm, and an accurate safety checking result and a market price signal are obtained. The method has important significance for improving the accuracy of clearing results of the power market and improving the safety level of the power grid.