Aiming at the requirements of shallow sea resource exploration and underwater environmental testing, a functionscalable modular ultrasmall ROV(Remote Operated Vehicle) was designed.The underwater robot had characters of modularity, small volume, light weight and good motion performance, mainly used in freshwater or shallow sea underwater detection and farming.The design and simulation of the sealed cabin of the ROV were focused on.The model of underwater robot body was established by using SolidWorks software. The strength of the inner cylinder part of the sealed cabin was checked under 100 m underwater through SolidWorks Simulation module. The results show that the wall thickness meets the safety requirements. The strength simulation checking of acrylic hemisphere cover with 6 mm wall thickness at the different depths of 50, 100, 200 and 400 m were carried out respectively. The results demonstrate that the maximum deformation of the acrylic hemisphere cover can reach 0.46 mm and the maximum stress is 29.2 MPa with the depth is 400 meters, satisfying the safety requirements.