Abstract: In-beam PET imaging provides an effective method to monitor the physical and biological dose distribution during carbon ion radiotherapy in real time. We investigate the dose distribution and In-beam PET image of a homogeneous water phantom and an abdominal tumor CT phantom by the Treatment Planning System(TPS) and Monte Carlo(MC) simulation. We compare the consistency of TPS dose distribution, the MC simulation dose distribution and the PET imaging profile at the tumor area. The relative biological effect values(RBE) were calculated using a linear quadratic model(LQ). Results show that the average error of the physical dose and RBE weighted dose in the water phantom and the abdominal tumor CT phantom were within 0.5% and 2% respectively. The carbon ion energy ranges from 120 to 400 MeV/u. The peak position difference between dose and PET imaging is within 8 mm. This paper proves the feasibility of using In-beam PET for dose monitoring during carbon ion radiotherapy.