Controlling the Schottky barriers (SB) in semiconductor/metal heterostructures is greatly vital to improve the performance of field effect transistors. Yet the modulation mechanisms have not been well understood. In this work, on the basis of first-principles calculations, we have demonstrated that both surface functionalization and external electric fields (E-ext) play significant roles in modulating the contact types (n-type or p-type) and SB heights at the interface by using the contact between MoS2 and Nb2C-based MXenes as a case study. The results show that weak van der Waals (vdW) interactions dominate between the interlayer. Importantly, the n-type SB-free contact is available in OH-terminated Nb2C/MoS2 heterostructure and continuously controllable p-type SB is found in the MoS2/Nb2CO2 and MoS2/Nb2CF2 heterostructures under proper vertical Eext strength. We unraveled that the modulation of Schottky barrier originates from the tunable metal work function induced by functionalized termination and the variable interface potential step induced by external electric field. Our work provides important clues for contact engineering and improvement of the MoS2 device performance.