Abstract: Spread spectrum ranging technology is an important approach for implementing TOA positioning,however,its ranging data with nonzero mean positive error will seriously influence positioning accuracy because of dense mine multi-path environments. To overcome this problem,a spread spectrum positioning method under the ranging plane constraint of projected mine tunnels is proposed based on the mechanism research on the generation of positive error. Firstly,a ranging plane coordinate system is established considering the extension direction of tunnel as x-axis and the width of tunnel as y-axis. As mine tunnel is a long and narrow chain-like space,the positional information of its x-di- rection is primarily concerned while its cross section can be neglected. So projected onto x-axis,moving target can be positioned in one dimensional space. By making a full use of redundant information of y-data,the position estimate of target within the designated zone of ranging plane can be constrained so that the accuracy of estimated x value is opti- mized evidently. The experiment results show that the proposed method is suitable to both NLOS and LOS situation. In addition,not only the zero mean error of estimate position in mine tunnel can be obtained,but also the random ranging error can be effectively restrained.