The self-collimation effect of TM mode in a two-dimensional square lattice photonic crystal is studied by using plane-wave expansion method, and two-dimensional finite-difference time-domain method with perfectly matched layer absorbing boundary conditions for the application of beam adjustment and deflection. The light transmission through the two-dimensional square lattice photonic crystal is investigated in detail with its equal-frequency contours plot and band structure, based on self-collimation effect and band gap in two-dimensional photonic crystals. The simulation results show that one-to-two beam splitter, deflector and one-to-three beam splitter can be realized by appropriately arranging the line defect along the proper direction. The energy of the transmitted beam and the deflected beam versus the radius of the line defect along ΓX direction is discussed. Such devices can greatly expand the application of photonic crystals in high-density optical integrated circuits.