Aimed at the problems of high filtrational resistance and low injection capacity in the development of high viscosity oil reservoirs with low permeability, the complex viscosity reducing system, composed of chemical heat generating agent and catalytic cracking agent, was proposed to reduce oil viscosity and improve oil recovery of the type oil reservoir. Laboratory experiment was conducted in high-temperature and high-pressure (HTHP) reaction kettle to optimize reaction conditions and catalytic agent type, and then the core flooding experiment was conducted to evaluate the effects of injection mode on the oil recovery and the injection capacity. The results showed that when the 200 mL of the NaNO2 solution with molar concentration of 4 mol/L reacted with 200 mL of the NH4Cl solution with molar concentration of 4 mol/L at the pH value of 2 in the reaction kettle with 400 mL volume, the temperature in the reaction kettle increased from 55℃ to 204℃, pressure increased from 10 MPa to 13.4 MPa after reacting for 6 min. The catalytic system, composited of oleic acid nickel (main catalytic agent), 7% formic acid (hydrogen donor), 7% urea (adjuvant) and 0.13% sodium dodecyl benzene sulfonate( emulsifier), could decrease the viscosity of the stock tank oil from Niujuanhu oil field by 65.3%. The oil recovery of water flooding, NaNO2 and NH4Cl alternating flooding, NH4Cl and NaNO2 alternating flooding, chemical heat generating and soaking interval flooding, and chemical heat generating and catalytic cracking and soaking interval flooding were 30.6%, 38.8%, 40.8%, 43.7% and 49.0%, respectively. Therefore, chemical heat generating and catalytic cracking and soaking interval flooding was the best injection mode, which could enhence oil recovery by 18.4% based on the water flooding.