The invention relates to a nanoparticle-added composite foam system used for an oil and gas field and a preparation method thereof. The composite foam system comprises components of, by mass, 0.3 to 0.5 parts of an anionic surfactant, 1 to 1.5 parts of modified silica nanoparticles, 0.03 to 2.3 parts of counter-ion salt, and 100 parts of water. The method comprises mixing the abovementioned components according to ratio, stirring by using a magnetic stirrer, standing, and then rapidly stirring to form well stable foam using a Waring Blender way. According to the composite foam system, the produced foam has a longer half life than that of foam stabilized by ordinary surfactant, and a larger start-foaming volume than that generated by adding a foam stabilizer The system has advantages of simple formula and preparation technology and good salt tolerance and temperature tolerance, can adapt to complex oil reservoir condition underground, does not pollute stratum, is capable of effectively blocking large channels and improving sweeping efficiency, and thus has good application prospects in oil field exploitation, especially in foam displacement.