Abstract: An in-house subchannel code SUBSC which was intended to be applied for large-scale thermal-hydraulics calculation was developed with object-oriented module coding technology. Both SUBSC and COBRA subchannel codes were used to calculate a typical PWR quarter assembly and the consistent results were obtained. In order to further validate SUBSC code, a steady-state bundle problem of the PSBT benchmark was calculated. The results show that the channel-averaged quality provided by SUBSC agrees well with the measured data at various conditions with the maximum relative deviation of 0.7%, demonstrating the accuracy of SUBSC code. In order to improve efficiency of SUBSC code, a restart GMRES algorithm with an incomplete LU factorization preconditioning was implemented for solving the mass conservation equation. The calculated results of multi-assembly indicate that SUBSC code is capable of dealing with large-scale thermal-hydraulics calculation.