The increasing energy demand stimulates the increase of the natural-gas-transmitting pipelines all around China and provokes serious fire risks as a result of accidental pipeline breaks and gas leakage, especially in cities at different altitudes. This work concerns the pressure effect on the flame height of buoyant jet diffusion flames restricted by parallel sidewalls in a reduced pressure, which does exist in natural gas leakage fire accidents in high-plateau areas. Experiments were conducted in Lhasa, Tibet, China (altitude, 3650 m; pressure, 0.64 atm), and the corresponding comparison results conducted in a normal pressure are referred from our previous work obtained in Hefei, Anhui, China (altitude, 50 m; pressure, 1.0 atm). The evolution of the flame heights of buoyant jet diffusion flames restricted by parallel sidewalls in a reduced pressure are examined, and the major new findings are that the evolution of a parabolic uprising buoyant vortex at the flame boundary and the flame heights vary much like that in a normal pressure as the sidewall separation distance increases from the minimum value. Moreover, the flame height in a reduced pressure is found to be slightly higher than that in a normal pressure, implying that a wider range of areas will be dangerous as a result of larger flames and exposure distances to radiation fluxes. The critical sidewall separation distance (S-cri) in a reduced pressure is figured out and correlated with the model obtained from our previous scaling analysis. Besides, it is found that, in a reduced pressure, the critical separation distance in a reduced pressure is slightly wider than that in a normal pressure, indicating that the surroundings should be located further to reduce fire risks. Finally, a global correlation accounting for the pressure effect based on the proposed model to characterize the variation of the flame height is obtained, correlating the experimental results in both reduced and normal pressures with good agreement. The current work can not only provide some supplemental knowledge on gas leakage flames restricted by surroundings in both reduced and normal pressures but also serve as a scientific basis to the management on the gas fuel energy storage and transportation systems in the cities at different altitudes to reduce possible fire threat.