The invention discloses a physical layer secure transmission method based on distrusted space modulation in a relay network. The method comprises the following steps that 1), when information transmission is carried out, a user node Alice randomly select Na relay nodes for carrying information of the user node Alice from all relay nodes and establishes a mapping function, wherein symbols of the information sent by the user node Alice and digital identifiers of the selected Na relay nodes are in one to one correspondence; 2), the relay nodes are divided into a relay set Omega1 and the relay set Omega2; 3), the relay nodes in the relay set Omega2 precode the received information and forwards the information to destination nodes; 4), receiving antennas in a spatial detection antenna set l1 counteract the information sent by the relay nodes in the relay set Omega2; and 5), the destination nodes decode the received information, thereby obtaining the information sent by the user node Alice. According to the method, the physical layer secure transmission of the information in a cooperative communication system can be realized without special relay nodes.