In extreme events, microgrid (MG) formation has drawn attention due to its potential to assist in load restoration in the distribution network by utilizing the distributed generations (DGs). However, most of the state-of-the-art studies pay attention to the steady constraints without considering the transient performance during MG formation process. Power fluctuations caused by line switch operations can lead to frequency overruns in low-inertia DG-based systems, thus tripping protective relays. This paper proposes an MG formation method for load restoration in the distribution network with dynamic frequency constraints during the load restoration process. Firstly, considering the frequency constraints, a frequency nadir formula is derived based on the aggregated model. The proposed MG formation method offers two solutions to ensure the frequency safety. One solution is to incorporate the dynamic frequency constraints into the MG formation optimization model to satisfy the frequency requirements if the load restoration amount is preferred. Another alternative solution is to introduce an inertia-adjustable control strategy using virtual synchronous generators (VSGs), which is aimed to improve the frequency nadir during MG formation process. This solution is implemented without changing the MG formation result that is subject to only steady constraints when the load restoration speed is privileged. Theoretical validity is verified through the simulation results. Case study results prove the effectiveness of proposed solutions under various demands in the aspect of frequency improvement.