Under the condition of open boundary, pedestrian evacuation dynamics in crossing exit is simulated through the lattice-gas model of biased random walkers without back step. Attention is concentrated on the mean velocity of pedestrian flow while simulating and analyzing, for it is the key factor to study the fundamental rules between the boundary density and width of route and to estimate jam transition during evacuation. In the simulation, the jamming transitions are observed at the longitudinal route or latitudinal route or both routes when the longitudinal pedestrian boundary density increases under the constant values of latitudinal pedestrian boundary density. Fixing the width of latitudinal route, a series of critical boundary densities which indicate the jamming transition are gained under different values of width of longitudinal route. By plotting the relation in certain coordinate system, the phase diagram can be concluded, which can express the jamming transition clearly. It is found in the phase diagram that the longitudinal and latitudinal dynamical phase transitions present different patterns. In the end, it should be pointed that the simulation results in this paper will be helpful for the design of evacuation routes.