We study the self-compression and splitting of a circularly polarized laser pulse propagating in plasmas with density window from 1/4 critical to slightly below critical density by solving the nonlinear Schrdinger equation numerically. It is demonstrated by the numerical calculation that the effective self-compression of laser pulse can be achieved in even shorter distance by increasing both the background plasma density and intensity of the laser pulse, or decreasing the width of pulse. The full-width at half maximum of the compressed pulse can reach 1/35 of the initial one's or even smaller. It has been found that this kind of self-compression occurs in the process of formation of a high-order soliton when a laser pulse propagates in a plasma, so that we can obtain greats compression ratio than in thin plasmas. We also obtained the splitting of a high-order soliton formed after self-compression of a laser pulse propagating in plasmas from the result of the numerical calculation in this situation. The phenomenon of self-compression and splitting is also observed by using one-dimensional particle-in-cell simulations and the result was consistent with the numerical calculation.