For operating CANDU reactors using on-power fuelling, the simulation results, which predict neutron flux and power distribution, are very important for economic and safe operations. To obtain better simulation results for operating CANDU reactors, a new simulation method is developed by using detector readings as a correction factor. The detector readings of CANDU reactors are directly used to correct the calculated flux distribution during iterations of core calculation. To describe the relationship between the flux at a detector position and the fluxes of mesh points around the detector, a suitable function is found, which is inversely proportional to the distance between the detector and the mesh point. The new simulation method is tested for the wolsung reactor(CANDU-600) using the fine mesh model through numerical calculations. The results show that the new simulation method can predict the core state more accurately with reduction of iterations than the present method. The new method improves simulation results by more than 50\% on the average compared to the present method, which does not use detector readings To reduce computing time, the coarse mesh model is introduced. Also, boundary conditions representing reflectors are applied for CANDU reactors. The attempt is successful and preduces better simulation results than PMCR which has been used for operating CANDU reactor. The simulation results using the coarse mesh model are improved by more than 70\% in comparison with those of PMCR. Since the new simulation method produces the excellent estimates of flul and power distributions, it can make lots of contribution to more operating margin of reactors and to optimization of fuel management. It can be also used to check detector readings of operating reactors.