To solve the problems of emulsification, viscosity reduction, and controllable demulsification of heavy oil, a pH-responsive heavy oil viscosity reducer, C16A-SiO2， was formed by the electrostatic interaction between cationic surfactant N,N-dimethylhexadecylamine (C16A) and SiO2 nanoparticles. And the performance and stability of the viscosity reducer were studied. When pH is 4.5 (±0.5), the viscosity reducer and the heavy oil can form a water-in-oil Pickering emulsion, the viscosity reduction rate for the heavy oil from Shengli Oilfield (viscosity of 5596 mPa·s，60 ℃) can be over 99.5%. The water separation rate is less than 20% after 8 hours. When pHis 8, the Pickering emulsion will breakdown rapidly. The structure of C16A-SiO2 was characterized by FTIR, TG, revealing a 27.3% grafting rate of C16A on SiO2. The Zeta potential, conductivity, surface tension, and contact angle were studied. The results showed that, when pH is 4.5 (±0.5), the viscosity reducer reveal the good surface activity, the surface tension of C16A-SiO2 aqueous solution dropped below 40 mN/m, the Zeta potential reached 42.5 mV, with a contact angle of 83.2°. The effects of pH, the mass fractions of C16A, and SiO2 NPs on the viscosity reduction rates and emulsion stability were studied. The optimal conditions for viscosity reduction were : pH is 4.5 (±0.5) , the mass fractions of C16A and SiO2 were 1.0% and 0.5%, respectively.