AbstractRecent advances in real-time spectral measurements of a mode-locked fiber laser have found many intriguing phenomena and which have verified the soliton theory. However, most current results are based on laser single-port observation, and are rarely involved in the cavity evolution, which also has rich nonlinear dynamics according to the soliton theory. Here we present an approach for the intracavitary soliton evolution processes, where spectra from multi-ports are collected in time-division multiplexed sequence to realize synchronous real-time observation. The sinusoidal evolution of the spectral beating is observed clearly, agreeing with the reported prediction. Furthermore, the intracavitary spectral dynamics of the period-doubling bifurcation are also revealed. Our scheme observed the spectral expanding and shrinking alternately and periodically over two round trips, matching well with simulations. This work may open up possibilities for real-time observation of various intracavitary nonlinear dynamics in photonic systems.