The HPR1000 unit reaches criticality with external neutron sources. The whole strategy is mainly formulated from the engineering experience, which lacks the quantitative supervision on the core subcriticality in the reaching criticality process. In this paper, based on the neutron diffusion calculation program of the PCM software package and the response function of the ex-core neutron detector calculated by the Monte Carlo code OpenMC, the inverse counting rate ratio of reaching criticality is simulated. Then the correspondence between the inverse counting rate ratio and the subcriticality is established. The effects of the location and intensity of neutron source, the core loading pattern and the starting and ending points of inverse counting rate ratio on the core subcriticality after the dilution are analyzed separately. Meanwhile, considering the variations of R rods bank value under different loading patterns, two strategies for the subsequent cycles to reach criticality are formulated from the aspects of safety and time saving.