The calculating methods for path number and path matrix of ventilation networks without unidirectional circuits were introduced, and the inapplicability problem of the matrix algorithm to determine paths in a ventilation network with unidirectional circuit was brought forward, and a depth-first search method based algorithm to determine path matdx by means of modifying search strategies was discussed. The method is suitable for ventilation networks with unidirectional circuits and ventilation networks without unidirectional circuits, moreover the complexity is less than the complexities of matrix based algorithms. A series of methods and means to avoid and to reduce the negative effects of unidirectional circuits such as the optimum of the positions of fan stations, the increase or de-crease of fan station numbers, the optimum regulation of network and the analysis of diagonal structures were put forward.