A new retarded acid system, nano homogeneous phase retarded acid was developed to solve the problems of high friction of emulsified acid and damage of gelled acid polymer residue in carbonate reservoir. The system consists of 20%HCL, 5% PA-NA nano emulsion, 0.1% drag reducer and functional additives. The properties of homogeneous phase retarded acid system were systematically analyzed and evaluated by means of rotating rock disc instrument, scanning electron microscope, EDS spectrometer, conductivity meter, particle size analyzer, interfacial tensiometer, wetting Angle tester, friction tester, acid erosion crack conductivity meter and core flow meter. The experimental results show that the homogeneous phase retarded acid system is a nano homogeneous dispersion system with oil in the inner phase and acid in the outer phase, and the particle size distribution is 8~100nm. It overcomes the disadvantages of high friction of emulsified acid and damage of thickened acid polymer residue, and has the properties of strong retarded speed (retarded rate greater than 90%), low friction resistance (lower resistance rate greater than 75%), low viscosity, low damage and high acid corrosion fracture conductivity. It can dissolve organic scale and inorganic scale at the same time (asphalt dissolution rate is greater than 5%, carbonate karst hydrolysis rate is 100%); It can remove the damage of reservoir emulsification, water lock and wetting modification. The homogeneous phase retarded acid system is adsorbed on the surface of positive electric limestone by negatively charged nano-emulsion to establish the "isolation barrier" of acid rock and achieve strong slow speed. Through differentiated acid erosion and viscous fingering, high acid erosion fracture diversion is realized. Through the special oil-water miscible structure of nano-emulsion, the organic scale and inorganic scale can be removed simultaneously. The successful application of this system in the field is of great significance for deep reservoir stimulation and multiple damage removal of carbonate reservoir.