In order to reveal the mechanisms of the inertial focus of buoyant particles in microchannel,the relative motion principle and numerical method were utilized to study the mechanical behaviors of a single spherical particle in rectangular microchannel with squared section,and some discussions were presented.The results show that in terms of particles with fairly small size,as the Reynolds number of flow increases to a certain extent,the inertial focus will appear, while the distribution of inertial lift on particles with fairly small size is instable on the lateral section of flow channel.Both the particle's rotation and the increased Reynolds number can lead the focus position to move towards the channel wall,while it will move away from the channel wall as the particle's size increases successively.The inertial lift consists of two components,the rotation induced lift and the non-rotation induced lift.The latter is the decisive component of inertial lift,which is composed by shear induced lift and wall induced lift.