The problems are discussed in positive pressure mechanical air supply system design, when the actual status is far different from what figures in the code based. Expressions must be used to determine the supply air rate. The rate between length and width of rectangle air channel should be less than 6. To avoid some uncertain factor, correction factor for absolute coarseness should choose the bigger figure. The smoke control and extraction design of smoke controlling stair hall must be divide into two parts: The overground and the underground. When setting positive pressure mechanical air supply system in smoke controlling stair hall, the outer window in the hall must be fixed. To insure the inner surface of smoke controlling stair hall keep compact and lubricity, communion should be done with governor of builders.