Abstract: The purpose of this paper is to study the critical sand starting velocity and transformation law of flow pattern based on gas–water–sand three-phase flow in an inclined pipe. Firstly, the indoor simulation experiment system of gas–water–sand three-phase flow was used to test the conversion law of flow pattern based upon the different gas void fraction. Secondly, the influence of slug bubbles on sand migration was investigated according to distinctive hole deviation angles, gas void fraction and sand concentration. Finally, the critical sand starting velocity was tested based on dissimilar hole deviation angles, gas void fraction, sand concentration and sand particle size, and then the influence of the above-mentioned key parameters on the sand starting velocity was debated based on the force analysis of the sand particles. The experimental results illustrated that when the gas void fraction was less than 5%, it was bubbly flow. When it increased from 5% to 30%, the bubbly flow and slug flow coexisted. When it was between 30% and 50%, the slug flow and agitated flow coexisted. When it reached 50%, it was agitated flow. Providing that the hole deviation angle was 90°, the phenomenon of overall migration and wave-like migration on the surface of sand bed was observed. On the contrary, the phenomenon of rolling and jumping migration was recognized. The critical sand starting velocity was positively correlated with the hole deviation angle and sand particle size, but negatively associated with the gas void fraction and sand concentration. This research can provide a certain reference for sand-starting production in the field of petroleum engineering.