The investigation of various soliton dynamics in a mode-locked fiber laser is briefly summarized, especially the recent progress of soliton transient dynamics in an ultrafast laser based on the dispersive Fourier transformation (DFT) technique is introduced. In addition, the multi-scale detection of ultrafast phenomena simultaneously using a time lens and the DFT technique is also briefly discussed, which helps to completely understand the physical nature of optical solitons. The development of real-time diagnostic methods can greatly inspire researchers to explore the soliton dynamics, and thus further promote the investigation of soliton dynamic characteristics as well as the development of ultrafast laser technologies.