Optical network-on-chip (ONoC) has been proposed to provide higher bandwidth and lower power consumption for future multicore systems. However, due to the intrinsic characteristic of optical devices, crosstalk noise is a potential issue for ONoC. The traditional routing algorithms for Benes ONoC have not considered the aspect of crosstalk. In this paper, we build the crosstalk model of the optical switch and analyze the crosstalk noise, signal-to-noise-ratio (SNR) and bit-error-rate (BER) for Benes ONoC. Moreover, we propose a routing algorithm to relieve the crosstalk problem. Simulation results verify the effectiveness of the algorithm compared to the conventional alternatives.