The PCM is capable of releasing and absorbing latent heat by changing its physical properties under a constant temperature to improve the performance of PV/T system, and thus the phase change melting of PCM within the collector deserves to be investigated in-depth. In this study, a novel PV/T-PCM collector by connecting the PCM with heat-absorbing plate in parallel is first proposed. Then, a mathematical model of the PV/T-PCM system is developed. Finally, the influence of key parameters on the phase transition melting of PCM along the flow direction within the PV/T collector is analysed. The results show that when the average solar irradiation intensity is 823 W/m2, 682 W/m2, and 569 W/m2, the complete melting rate of the PCM at five positions within the collector is the same, with a maximum difference of 10% and a maximum difference of 5%, respectively; under the same preconditions, the maximum time difference in the phase transition start time is 62 mins, 72 mins, and 93 mins, respectively. When the average ambient temperature is 37 °C, 28 °C, and 16.6 °C, the complete melting rates of PCM at five positions is also the same, also with a maximum difference of 10% and a maximum difference of 5%, respectively; under the same preconditions, the maximum time difference in the phase transition start time is 60 mins, 68 mins, and 73 mins, respectively. When the water tank volume is 65 L, 85 L, and 105 L, the complete melting rates of PCM at five positions are the same, with a maximum difference of 5% and a maximum difference of 10%, respectively; under the same preconditions, and the maximum time difference in phase transition start time is 47 mins, 60 mins, and 77 mins, respectively. When the water mass flow rate is 0.006 L/s, 0.018 L/s, and 0.036 L/s, the complete melting rate of PCM at five positions within the collector has a maximum difference of 15%, 10%, and 5%, respectively; under the same preconditions, the maximum time difference in the phase transition start time is 180 mins, 74 mins, and 28 mins, respectively.