In order to take advantage of the flexible adjustment ability with new energy in unit combination and make up for the lack of system inertia caused by new energy access，a unit combination model is constructed in this paper，which takes the inertia support and primary frequency regulation capabilities of grid-forming new energy. The model incorporates dynamic frequency constraints and grid-forming control parameters，utilizing the shedding capacity stored through new energy curtailment. Then，the nonlinear mixed integer programming model is transformed into a multi-objective two-layer optimization problem for iterative solution. Thus，the optimal start-up mode of synchronous generators，the optimal reduction of new energy and the corresponding control parameters are obtained which meet the system frequency constraints. A high proportion of wind power transmission grid in Gansu province is taken as an example to verify the method. The results show that compared with the existing scheme that only considers frequency constraints and the unit combination scheme with constant grid-forming control parameters，the unit combination scheme with variable grid-forming control parameters proposed in this paper can further improve the system operation economy and new energy utilization rate under the frequency security constraints.