The irregularly shaped distributed Bragg reflector (IDBR) for thin film solar cell is designed according to the theory of multi-beam interference, contains two pairs of α-SiH(36.5 nm)/SiO2(81 nm) distributed Bragg reflector (DBR) and three pairs of α-SiH(73 nm)/SiO2(162 nm) DBR. Different structures of back reflector are discussed. The criterion to choose the central wavelength and pairs of DBR is analyzed. The method of iteration is used to calculate the reflectivity. The IDBR is fabricated by plasma enhanced chemical vapor deposition (PECVD) on a silicon wafer. The experimental result shows that the average reflectivity of IDBR is 87% in the spectral range of 420~1400 nm, and is as high as 91.6% from 600~1300 nm. The result indicates that this design has the potential to enhance the light absorption for thin film solar cells in broad band.