In this study, pull-off bond tests and four-point bending tests are performed for carbon strand sheet (CSS) strengthened concrete beams, to evaluate the short and long-term performance of CSS/concrete interfaces. The focus is on the influences of highly moist climate during FRP implementation and the following service period. It is found that high R.H. in air during the curing of bond lines has a marginal effect on the bond performance of the CSS/concrete interfaces. However, if the concrete substrate is wet before bonding the shear failure of the CSS/concrete interface will shift into the primer/concrete interface or a very thin mortar layer, which is different from the conventional failure mode observed in a dry substrate case. Fortunately, use of a hydrophobic type of primer can prevent the interfacial bond line from the tensile strength loss caused by the wet substrate. For the CSS/concrete interfaces that have been subjected to an 8-month accelerated cyclic dry/wet exposure, microscopic observations reveal that micro-cracks are formed at the primer/concrete interface during the exposure and lead to a significant loss of the interfacial pull-off bond strength. On the other hand, flexural tests on the CSS strengthened concrete beams indicate interestingly that the shear bond force transfer capacity of the CSS/concrete interfaces does not decrease while the interfacial bond deformability increases greatly after the exposure. The critical problem is the moist-induced shear stiffness loss. These above experimental findings provide useful information for predicting the service-life performance of CSS strengthened concrete members under a cyclic moist climate.