The present invention discloses a ternary composite system suitable for high-temperature and high-salinity oil reservoir, and a preparation method thereof. The ternary composite system comprises a polymer, a surfactant and sodium carbonate, wherein the chemical structure formula of the polymer is defined in the specification, and the polymer has a network structure and has the effect of effective inhibition of high temperature and oxidation degradation so as to effectively improve salt resistance and strengthen thickening capacity. According to the present invention, after the polymer is introduced into the ternary composite system, the ternary composite system has good chemical stability and excellent salt resistance under the conditions of high temperature and high salinity, makes the crude oil/stratum water interface tension be reduced to an ultra-low level under the weak base sodium carbonate condition, and can improve the recovery rate by more than or equal to 20%, such that the ternary composite system of the present invention can be used for the extreme high-temperature and high-salinity oil reservoir conditions.