The invention belongs to the technical field of free radical detection, and provides an in-situ online detection method for hydroxyl free radicals in a
hydrocarbon-water
system high-temperature action process. The method comprises the following steps: preparing a capillary reaction kettle, preparing a capillary
injector, preparing a
test solution, and injecting a sample: sequentially injecting an
aqueous solution and a
hydrocarbon mixed solution containing a paramagnetic
trapping agent into the capillary reaction kettle, so that the
hydrocarbon mixed solution is discontinuous in short-column-shaped sections in the
aqueous solution, a'hydrocarbon-water-hydrocarbon-water-hydrocarbon-water... 'multi-hydrocarbon-water 'interface is completed; sample detection and
image analysis. According to the invention, hydrocarbons and a water solution to be detected are sealed by using a
fused quartz capillary tube and a high-pressure valve, and multiple sections of hydrocarbon-water interfaces are sealed in a
quartz capillary tube
system, so that the oil-water interfaces of the hydrocarbon-water
system in the capillary tube are densely arranged near detection points; the distribution density of oil and water droplets near an interface under a high-temperature condition and the generation of a large amount of free radicals are increased, and the
signal-to-
noise ratio of effective signals is improved.