In order to investigate the characteristics of dichromatic laser in preparing coherence population trapping (CPT) states in CPT atomic clock and CPT magnetometer, thermal 87Rb atoms are sealed with buffer gas in a glass bulb are prepared into CPT states with dichromatic coherent radiation field. We attribute all states without direct interaction with bi-chromatic coherent radiation field as a “leaky trap state”, and describe the system with a simple four-level system. Through solving optical Bloch equations of the four-level system, we have theoretically studied the preparation of CPT states, especially the de-coherent effects by thermal collision on the two ground states of the Λ structure. The results reveal that this de-coherent collision not only decides the rate of prepared CPT states, but also causes the maximal coherence deviating from the equal intensities of bi-chromatic components. A simple expression is given for roughly estimating the influences of de-coherent collision and laser intensity.