For offshore floating nuclear power plant the complex ocean conditions must be considered for safe design and review. And the neutron kinetics decides the power distribution and variation. Both of them will change the characteristics of the core instability. In this paper the ocean condition mode and diffusion models are added into the parallel channel system thermal-hydraulic model. An imaginary 36-channels reactor core is selected as research object. The stable and unstable cases of the system are analyzed firstly. The oscillation characteristics of mass flow rate and total power are studied. Then the instability boundaries under ocean condition and neutron feedback are obtained, which is compared with the inherent boundary. The results indicate that the combined effect have powerful influence on the system instability and should be noticed. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.