The optimum process of extruded instant porridge was studied with composite grain powder as raw material. Based on the previous single factor experiment results， the process of extruded instant porridge was optimized by box Behnken design response surface method. The quality of extruded instant porridge was evaluated by taking physical and chemical properties， sensory score， texture properties and digestibility of protein starch as investigation indexes. The structure of instant porridge before and after extrusion was studied by Fourier infrared spectroscopy and X-ray diffraction. The results showed that the best technology of extruding instant porridge was as follows: Screw speed 250 r/min， extrusion temperature 170 ℃， water addition 19%. In the validation test， the coefficient of variation is between 0-12%， and the optimized process conditions of instant porridge have good reproducibility， indicating that the process parameters optimized by response surface are reliable. Under these conditions， The protein digestibility and starch digestibility of the instant porridge increased by 12.570% and 12.602%， respectively， after extrusion compared with that before extrusion. Fourier transform infrared spectroscopy analysis showed that the structure of main functional groups did not change significantly before and after extrusion， and the intensity of some characteristic peaks decreased after extrusion. X-ray diffraction scanning analysis showed that the crystal configuration of instant porridge changed before and after extrusion. After extrusion， the quality of instant porridge was improved and the microstructure changed.