Abstract: A method for determining the constituent content of a multiphase fluid includes the following steps: x-rays at single-energy or dual-energy levels are produced by an x-ray machine, after said x-rays pass through the multiphase fluid, the data at each energy level are detected by a detector sub-system which is composed of one or two detectors, and the mass percents of the components in the multiphase fluid are calculated by a controlling and data processing sub-system based on the detected data. Said multiphase fluid is a two-phase or three-phase mixture in crude oil or natural gas. The method can be used for automatic online measurement of the production in oil and gas fields.