This paper studies the vibration analysis of the cylindrical nanoshell reinforced with carbon nanotubes (CNTs) conveying viscous fluid in the hygro-thermal environment for the first time. Nonlocal strain gradient theory (NSGT) is applied to consider the small-scale effect, including both softening and stiffness impacts. First-order shear deformation theory (FSDT) is utilized to study the shear impacts. A general nonlinear relation is applied to study the hygro-thermal effects. To consider the fluid–solid interchange in several environments, Navier–Stokes equations are employed. Hamilton’s principle is utilized to achieve the governing equations of motion. Then, Navier analytical method is applied to derive the natural frequency of the nanoshell with simply-supported boundary conditions. The impacts of several variables are investigated on the vibration characteristics of the CNT cylindrical nanoshell.