The safety of electromagnetic exposure induced by 5G base station has become a big issue accompanied with its large-scale commercial application. To solve the assessment of 5G base station equipped with massive multiple-input multiple-output (MIMO) and 3D beam forming technology, this paper proposes an improved statistical approach based on the previous Monte Carlo method to evaluate the electromagnetic field (EMF) exposure. As a studied case, a three-dimensional user scenario located in urban area is built to demonstrate the implementation process of this novel method. The result shows that the proposed relative EMF exposure factor is less than 25% when assuming the worst-case scenario, same equivalent radiated power and mounting height for both the 5G and 4G base station. Moreover, when comparing with the 4G base station, a reduced and uniform field distribution is obtained due to the random beam direction and time average in 5G base station.