Within the framework of the ray optics theory, the effects of heating effect and geometric aberration on optical quality of fiber laser beam are calculated and analyzed. An analytical expression for the beam quality parameter M2 of fiber laser beam propagating through a multilens optical system is derived as a function of the system parameter. The optical quality of high-power laser beam propagating through a collimating system is investigated. It is shown that a best optical quality can be found as the laser power increases and the corresponding laser power depends on the collimating system parameter. The investigation presents a reference for the design of high-energy fiber laser collimating systems.