In order to simulate the explosion process due to Liquefied Natural Gas(LNG) pipeline leakage in tunnel, a mathematical model of fire and explosion due to LNG pipeline leakage is proposed based on the Computational Fluid Dynamics (CFD) method and for an active LNG transportation pipeline in a tunnel. The LNG leakage explosion under three different LNG leakage conditions are numerically simulated, and their overpressures at the measuring points are obtained for tunnel with solid ends and pressure release ends. The explosion overpressures caused by LNG pipeline leakage under three different boundary conditions are obtained and the destruction effect is evaluated. The simulation results show that the explosion is in the form of deflagration when the LNG leakage intensity is the maximum or the minimum in the tunnel with solid ends or pressure release ends, which would damage badly the equipments in tunnel. When the LNG leakage intensity is a medium one, the deflagration would transform into the detonation with very strong damage power, and this should be avoided to be happening.