Embodiments of the present invention disclose a path selecting method and apparatus. The method includes: computing an end-to-end path for a newly added service according to network topology and a wavelength constraint, and assigning a wavelength to the path; computing performance of each existing service and performance of the newly added service in a network according to physical impairment information collected in the network, where the physical impairment information includes a gain reference spectrum of each optical amplifier in the network; and performing impairment check on performance of each service, and performing path selection for the newly added service according to a result of the impairment check. The apparatus includes a path computation module, a performance computation module and an impairment check module. According to the embodiments, efficiency of network update or rerouting is improved.