Hybrid precoding is a credible solution to address the trade-off between spectral efficiency and power consumption in mmWave massive MIMO systems. Though traditional hybrid precoding with fixed architectures has reached its limits, the flexibility of hybrid precoding has not been fully exploited, which gives insight to improve the performance further. In this paper, we adopt a novel architecture called dynamic-subarray, where the connection between radio-frequency (RF) chains and antennas can be dynamically changed by switches. We propose a low-complexity scheme to design the hybrid precoder with dynamic subarrays. Simulation results demonstrate that our proposed method has an excellent performance on the achievable rate and energy efficiency even with lower complexity.