In this paper, an analytical threshold voltage model is proposed for a triple-material cylindrical gate-all-around MOSFET considering parabolic approximation of the potential along the radial axis. The center (axial) and the surface potential models are obtained by solving the 2-D Poisson's equation in the cylindrical coordinate system. This paper refutes the estimation of the natural length using surface potential as in previous work and proposes the use of center-potential-based natural length formulation for an accurate subthreshold analysis. The developed center potential model is used further to formulate the threshold voltage model and also extract drain-induced barrier lowering (DIBL) from the same. The effects of the device parameters like the cylinder diameter, oxide thickness, gate length ratio, etc., on the threshold voltage and DIBL are also studied in this paper. The model is verified by the simulations obtained from 3D numerical device simulator Sentaurus from Synopsys.