Monolayer black phosphorous (BP) is a commonly used semiconducting two-dimensional material, thanks to its unique electronic properties of an atomically thin two-dimensional layered structure and its ability to be applied in a novel nanoscale metal-oxide-semiconductor field-effect device. In this paper, we focus on a new compact band structure model and apply it to electron mobility calculations for monolayer BP. We propose a new compact band model based on an effective mass approximation considering a second-order non-parabolic correction to calculate the band structure, density of states, velocity squared, and other physical quantities of monolayer BP. Electron mobility using the Kubo-Greenwood formula is calculated based on the new corrected compact band model. Our new compact band model is very useful to TCAD simulation and design of future transistor devices with monolayer BP.