Focusing on the survivability of long-reach passive optical network against single shared-risk link group failure, a cost-efficient planning approach based on connection availability was proposed. First, a connection availability model based on failure probability was designed to estimate the connection availability of each optical network unit. Each primary optical network unit not satisfying the connection availability requirement would be allocated some backup optical network units, each of which needed to reserve the backup capacity for the primary optical network unit. Then, deployed the backup fibers among different optical network units to establish at least one backup-optical-path between each pair of primary and backup optical network units. Through the computational simulation, the performance of proposed approach in deployment cost of backup fibers was analyzed. Results show that the proposed approach can achieve much lower deployment cost than the previous neighbor protection approach. The proposed approach would solve the problem of the joint optimization of backup capacity allocation and backup fibers deployment. The objective was to fully protect all traffic demand in the network by the minimum deployment cost of backup fibers, while guaranteeing the connection availability requirement.