Millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) systems typically employ hybrid beamforming architectures to reduce the number of radio-frequency (RF) chains, thereby the cost and power consumption is decreased. Compared with fully connected architectures, subconnected architectures provide greater energy efficiency in general. In this paper, we devote to find the optimal subconnected hybrid structure. We first proposed a novel model to describe the connections between antennas and RF chains, based on which the optimal problem is formulated and solved by considering the channel characteristics. Meanwhile, the factors affecting the spectral efficiency of diverse subconnected hybrid structures are analysed. Simulation results show that the spectral efficiency is related to the inter antenna spacing. When the inter antenna spacing equals half wavelength, the optimal interleaved scheme is given in closed form.