With the access of high proportions of new energy，synchronous generators are gradually phased out. On one hand，system inertia is reduced. On the other hand，during short-circuit faults，the low-voltage ride-through characteristics of new energy may result in short-term power disturbance，leading to rapid drops in grid frequency. Frequency emergency control，as a critical measure to safeguard frequency stability after grid faults，may encounter issues such as delayed actions or improper control actions resulting in high or low frequency events. To address these challenges，a frequency response model incorporating frequency emergency control is established. The boundaries of emergency control effectiveness under typical conditions are studied. Furthermore，a frequency response model that considers both frequency emergency control and grid-forming new energy is developed. Based on this model，the influence of various frequency control parameters，such as power reserves of new energy and droop control coefficients，on the effectiveness of frequency emergency control is analyzed comprehensively. Requirements for frequency regulation performance of grid-forming new energy to ensure effective emergency control are provided. Finally，the frequency model and the proposed effectiveness boundaries of emergency control are verified using simulations on the IEEE 10-machine 39-bus system. The results show that the participation of grid-forming new energy in frequency regulation can effectively extend the operating boundaries of emergency control，and its power reserve has a significant impact on the upper and lower boundaries of frequency emergency control effectiveness.