The increase of grid load peak-valley difference and the development of trans-regional power transmission lead to the peak load regulation problems in valley periods of load. An evaluation model of peak load regulation capability in valley periods of load is established, which optimizes the times of units’ startup and shutdown and the minimum downward adjustable power during the research period considering various types of constraints of unit operation and grid operation and achieves the quantitative assessment of negative peak load regulation capability. On the basis of otherness of peak load regulation capability between two grids, a trading mechanism is proposed and two trading modes including valley trade and peak-valley-bundled trade are analyzed. Then a method to determining the regulation demand of lacking-load grid is proposed on the basis of evaluation model of peak load regulation capability. The results of typical case indicate that the proposed model can effectively evaluate the peak load regulation capability and the proposed trading mode for trans-provincial aid is feasible and effective. This work is supported by Science and Technology Project of State Grid Corporation of China (No. 520800150014).