The disjoining pressure has a significant influence on the stability of thin films, which is important in various biological, environmental, and industrial processes. However, due to its complicated nature, especially in nanoscale, a convenient way is still lack. Here, a methodology derived from film thermodynamics was developed to determine the disjoining pressure of thin films using molecular dynamics (MD) simulations. The obtained disjoining pressure was purely attractive for pure water films, and compared with that from the Hamaker constant by Lifshitz-van der Waals theory. This method can be readily applied to similar aqueous films, which will enhance the interpretation of laboratory experiments and extend previous theoretical framework.