The photonic band gap (PBG) of eight-fold photonic quasicrystal (PQC) with dielectric cylinder structure is studied using the finite difference time domain (FDTD) method, and the effect of disorder of cylinder size and displacement of cylinder sites on the width, site and anisotropy of photonic band gap is numerically investigated. When the disorder of cylinder size and displacement of cylinder sites increase, the central frequency of the photonic band gap has a redshift, and the relative band gap width decreases. The band gaps are much more sensitive to disorder of cylinder size than displacement of cylinder sites. When the disorder of cylinder size is half the cylinder radius, the band gap disappears completely, but the band gap only changes 1% for the same size of displacement of cylinder site. The isotropy of band gap is maintained in a large range of structure disorder.