Aiming at a MV microgrid containing multi DERs, a PSO optimal power flow algorithm based on spot power prices is detailed. Firstly, the economic cost formula, which takes interruptable loads and bi-directional current into account, is deduced. Meanwhile, the battary operation constraints considering energy loss are given. To minimize the 24-hour operation cost, a general economic dispatch model for microgrid is presented. The model considers generation cost, emission cost, health cost (the influence caused by PM10) and various constraints. Secondly, a chaos PSO algorithm based on Newton-Raphson optimal power flow method is proposed. The 24-hour load curve after load forecast, together with spot power price in a power market, is adopted to realize economic power share among interruptable load, battary, DER and main power grid. Finally, the program of PSO optimal power flow algorithm is developed in Matlab environment. Through analyzing the economy of a MV microgrid under four conditions, the correctness and validity of the method is proven.