The spray characteristics of biodiesel and diesel were studied in a constant volume combustion bomb by using high speed photography under high injection pressure. The effects of background pressure, background temperature, and injection pressure on the spray characteristics were studied by means of CFD code. The results indicated that the penetration of biodiesel decreased with the increase of backpressure; penetration of biodiesel increased with the increase of injection pressure; with the increase of background temperature, penetration of biodiesel began to fluctuate due to the high evaporation rate.