We propose a two-dimensional photonic crystal structure with low rotational symmetry and investigate its band structure characteristics over the whole first Brillouin zone by the plane wave expand method. The physical mechanism of broadband all-angle self-collimation effect and the influence of aspect ratio on the bandwidth are clarified. Furthermore, we obtain the existence criterion for self-collimation modes covering almost the whole fourth band for TE polarization. Especially, there exist two wide flat regions spanning over the first Brillouin zone which exhibit unique properties: one dimension corresponds to broad band from 0.47 to 0.53 (2πc/a), while the other corresponds to all incident angles of 0°–90°. Based on the above unique properties, the broadband all-angle self-collimation propagation with a bandwidth of 187 nm around 1550 nm is demonstrated by the finite-difference time-domain method.