Abstract: Monte Carlo method is extensively employed in radiation shielding calculation with the advantages of high fidelity geometry modeling, complex radiation source description and continuous-energy cross sections. However, the simulation is impractical for Monte Carlo particles that have little chance of being transported to the far-source area and the statistical error may be unacceptable. Accordingly, effective global variance reduction method (GVR) is significant for Monte Carlo deep-penetration radiation shielding calculation. This work constructs weight window parameters based on forward Monte Carlo calculation to bias the particles position and energy during transport. According to the validation of shielding benchmark, particles have been guided to far-source area by using the weight window. The relative error has been decreased to less than 10%, and almost all the mesh cells have been tallied. The results show that the transport parameters obtained from forward Monte Carlo calculation could realize global variance reduction.