A Janus particle is a general term for a non-uniform particle that has different properties on different sides of particle. For a Pt-SiO2 type of Janus microsphere, Pt side serves as the catalysis surface to decompose H2O2 solution, leading to the self-propulsion motion of particle. In this paper, the relevant experimental phenomena in two driven modes are compared first. The results show that under the same concentration of solution, the microsphere with a diameter of about 1 m experiences self-diffusiophoresis propulsion; whereas, the one with an about 20 m diameter experiences bubble self-propulsion. Significant differences in motional trajectory and propulsion velocity are found between them. Then, the dominated physical factors are analyzed and the multi-field coupling numerical model is constructed based on the simplified force balance analysis. Subsequently, the velocity field distribution and O2 concentration distribution around Janus microsphere are also studied. According to these studies, we explain the position and size of the bubble generated. Further more, we infer that the wall slip coefficient is a key matching parameter in the numerical model, and two slip coefficients with a difference of an order of magnitude are given corresponding to the two types of self-propulsion modes. Then we explain the possible mechanism for the changes of wall slip coefficient under different particle sizes. The present study is beneficial to the in-depth exploration of the self-propulsion mechanism and also provides the theoretical foundation for improving the performance of self-propellant device.