Abstract: In order to evaluate the influence of proppant on the conductivity of acid-etched fractures during the composite acid fracturing with sand and determine the timing of adding sand, an evaluation method for the conductivity of sand-packed acid-etched fractures at full-length scale was proposed. The Ordovician reservoir in Shunbei Oilfield was taken as an example. Firstly, acid fracturing numerical simulation was used to determine the key test conditions such as temperature, mass fraction of acid solution, and sand concentration at different positions of fractures under reservoir conditions, and then the conductivity of sand-packed acid-etched fractures at different positions was tested. The results showed that the impact of reaction temperature at different positions of the fractures on the acid-etching effect was greater than that of the mass fraction of acid solution, which led to the best acid-etching effect in the middle of the fractures, followed by the fracture tail, and the fracture inlet was the worst. When the closure stress was greater than 60 MPa, sand filling in the middle and tail of the acid-etched fractures could significantly improve the conductivity. This method was applied in Well S3 to determine the sand addition during the metaphase of acid fracturing, so as to improve the conductivity in the middle and tail of the fractures. After the stimulation, the stable daily oil production was increased by 40% compared with the adjacent wells, and the stable production time was extended by 57.8%. The evaluation method for the conductivity of full-length sand-packed acid-etched fractures overcomes the difficulty to evaluate the conductivity of fractures with the order of 100 meters under reservoir conditions which is a limitation for conventional tests and provides a new method for determining the timing of adding sand in composite acid-fracturing.