Different from previous focusing on subwavelength imaging in the near field, we outline another potentially useful application of the self-collimating photonic crystal -- low-pass spatial filtering. A two-dimensional square-lattice photonic crystal is taken as an example and the low-pass spatial filtering is demonstrated by the dispersion analysis and numerical simulations. The high-spatial-frequency components, whose incident angles exceed a critical value, are reflected totally because no Bloch modes of the photonic crystal can be excited. However, the low-spatial-frequency components are coupled to the self-collimating modes and permeate the photonic crystal with high transmissivity.