A self-collimation splitting sensor truncated in a two-dimensional (2D) photonic crystal has been proposed and demonstrated theoretically. Intensity of transmitted light and reflected light, which varies with the width of the splitter, is detected at two output ports. Calculation results validated by the 2D finite-difference time-domain technique show that with the increase of width, the transmittivity decreases while the reflectivity increases and their summation decreases. Considering the trade-off between the sensitivity of the optical detector and the loss of power, including the difficulty of fabrication, the width of 0.4a is meet for the sensor. The simulated relative intensity monotonously increases from 1.63 to 2.94 nonlinearly.