The dispersion behaviour of leaking hydrogen in a partially open space is simulated numerically by a balancing domain decomposition method in this work. An analogy of the thermal convection problems is applied. The linear systems of Navier-Stokes equations and the convection diffusion equation are symmetrized by a pressure stabilized domain decomposition method. The interface problem after domain decomposition is solved by a balancing preconditioned conjugate gradient method. Numerical results are validated by comparing with the experimental data and available numerical results. The transient behaviour of hydrogen dispersion and accumulation in the partially open space is discussed; the mechanism of air influence is investigated and safe region inside this partially open space is classified. (C) 2015 Elsevier Ltd. All rights reserved.