Waterproofing is one of the key problems of underground structures. Spray-on waterproofing membrane is a new system in tunnels with single shell linings. The conditions of the application of this system are analyzed in terms of the deformation and surface condition of the first layer of the shotcrete shell, water ingress and water pressure behind the waterproofing membrane. The results show that the applications of a spray-on waterproofing membrane in a tunnel is possible if the deformation of the shotcrete and water pressures behind the sprayed membrane is properly controlled, as well as the ingress water is piped or tightened with practical measures. Proposals of measures to meet the conditions are presented for the application of the spray-on waterproofing membrane in tunnels.