Abstract: A spatiotemporal mode-locked laser with a central wavelength of 1065 nm and a pulse repetition frequency of 25.8 MHz is experimentally built based on the nonlinear polarisation rotational mode-locking technique. And a passive multi-mode fiber with core diameters of 50 μm and 62.5 μm is fused to the gain fiber to construct a laser to study the variation of the pulse characteristics with the core diameter, respectively. In both cases, the fiber laser is mode-locked, and the output power, state evolution, beam profile and spectral characteristics of the pulses are measured and compared, and the comparison results show that the spatial mode complexity of the mode-locked pulses is proportional to the core diameter of the fiber. With a core diameter of 62.5 μm, a similar self-similar spatiotemporal mode-locked pulse output is found by optimizing the spatial filtering effect, and the spatial spot is transformed from a spurious multi-mode state to a Gaussian output close to the fundamental mode, corresponding to a significant increase in the pulse energy, which provides a new platform for the construction of high-performance and high-power mode-locked fiber lasers.