A multilink failures model, i.e., probabilistic-shared risk link group (PSRLG), is adopted to investigate the problem of differentiated quality-of-protection (QoP) provisioning for flexi-grid optical networks. As a metric, service failure probability (SFP) is introduced to exactly examine the feasibility of differentiated QoP schemes, which denotes the failure probability of a connection during transmission. According to different reliability requirements, connection requests are divided into three classes, i.e., class high, class middle, and class low. Then two differentiated QoP provisioning schemes are proposed based on the class division, i.e., intraclass-shared resource scheme (ICSR scheme) and cross-class-shared resource scheme (CCSR scheme). The former allows a connection to share backup resources only with those connections in the same class, whereas the latter enables the connections in different classes to share backup resources. Simulation results show that our proposed schemes could well provide differentiated reliability with PSRLG constraint and achieve a good balance between reliability and resource efficiency. Moreover, the CCSR scheme achieves lower blocking probability, lower resource redundancy, and higher spectrum utilization without sacrificing reliability compared to the ICSR scheme.