The invention discloses a method for determining gas-liquid two-phase hydrodynamic slug flow in horizontal and micro-inclined pipes. The method comprises the following steps of: 1) on the basis of a drift flux relational expression, acquiring a liquid-phase phase volume fraction HL-EXP based on an experiment result, improving the expression of drift speed U0, wherein improved U0 is a function of liquid phase converted speed USL; 2) according to a relational expression indicative of a change rule of the U0 to the USL, drawing a curve of the drift speed U0 to the liquid phase converted speed USL, and determining the gas-liquid two-phase hydrodynamic slug flow according to the form of the curve, wherein a specific determining criterion is that when the drift speed U0 increases with an increase in the liquid phase converted speed USL, the flow in a pipeline is stratified flow; when the drift speed U0 reaches an inflection point from increase to decrease, the flow in the pipeline is changed from the stratified flow to slug flow; in the decrease segment of the drift speed U0, when the curve reaches an inflection point where the linetype of the curve changes, the flow in the pipeline is changed from the slug flow to bubble flow.