Pattern formation is closely related to system boundary conditions in nonlinear dynamic systems, apart from the system control parameters. To avoid complexity, system boundary conditions are usually considered to be infinite or periodic and the initial conditions spatially homogeneous. But it is not always the case in real situations, or sometimes periodic boundary conditions are not exact. To show the important and interesting boundary effects in real pattern formation, we suggest a simple universal boundary condition in a typical optical pattern formation system. Numerical simulations of the passive optical system show that pattern characteristics such as distribution symmetry, peak number, structure strength, evolution course and stability are all greatly influenced by the system boundary conditions.