The vehicle-mounted forward-looking ground penetrating SAR(VMFL-GPSAR) is a kind of bistatic SAR. Fast factorized backprojection(FFBP) originally developed for unistatic SAR fails to efficiently form well-focused VMFL-GPSAR image. A bistatic FFBP algorithm is derived after the analysis of the round trip range error of FFBP in VMFL-GPSAR. In the bistatic FFBP, the range error can be reduced by rational subaperture center configuration and by polar angle interval constraint. Besides, by combining range phase compensation and 2D linear interpolation, the bistatic FFBP can perform faster with the same focus precision. Simulation and VMFL-GPSAR raw data processing validate the bistatic FFBP algorithm, achieving a 2.31 times speedup.