Abstract In solving the crystal structures containing heavy atoms by patterson method, if the heavy atoms have a pseudo-symmetry which is higher than that of the whole structure, this will result in multi-solution positions of light atoms (two, four or eight possible positions). Expressed in reciprocal space, this problem is equivalent tc that some of the component signs of the diffraction structure amplitudes are undetermined. Seventy four space groups of low symmetry systems (trielinic, mono-clinic and orthorhombic systems) have been analysed. For an asymmetric unit involving only one heavy atom, with the possible positions of the heavy atom and the pseudo-symmetry given, the multi-solution types of light atoms and the distribution rule for components of diffraction struture amplitudes are obtained. In addition, a possible way for resolving these problems by direct method is proposed.