The physical-layer security of coherent time-spreading optical code division multiple access (OCDMA) system is analyzed quantitatively from the view of security of information theory. Relationships between the system security capacity and different system parameters are obtained. The wiretap channel model of OCDMA system is established, and the system security capacity is calculated. Influences of key system parameters (user number M, input optical power P and code length N) on the physical-layer security of coherent time-spreading OCDMA system are examined by using Gold code as an example. Results show that the coherent time-spreading OCDMA system has a very good physical-layer security against fiber tapping attacks, and the above key system parameters have important influences on improving the system physical-layer security. It can improve the security of coherent time-spreading OCDMA system by choosing the appropriate system parameter values.