M7 block of Santanghu oilfield is an abnormal low temperature tight oil reservoir. The effective development is very difficult. In this paper, we develop a new composite viscosity reducer called CSY-1. The synergistic effect of these agents provides the reducer with both dispersion and emulsification capacities. Based on orthogonal experiment and cost evaluation, the optimum formula of CSY-1 is 0.5% OP-10, 1.8% SDBS, 0.1% Tween 80, and 1% NaOH (or Na2CO3). The rheology property of heavy oil changed fundamentally before and after the addition of CSY-1. Oil-in-water emulsion with very low viscosity can be obtained and the viscosity reduction rate can be over 99%. Simulation results showed that the oil production of a single well can be increased from the current 3.74 t/d to 8.12 t/d with the application of this reducer.