The invention provides an inclined pipe type H-shaped on-line separation method for underwater crude oil. The method is applied to underwater crude oil on-line separation. A separation system organically combines vertical type and horizontal type on-line separation of the crude oil with a high-voltage electric field rapid dehydration process and a control system thereof, and according to two-stage series, inclined pipe type structure and H-shaped arrangement, two-stage rapid separation of the underwater crude oil is achieved. A first-stage pipe type separator conducts the first stage of inclined pipe type oil-gas-water pre-separation process, and wet gas and most of water are separated out. A second-stage pipe type electric dehydrator adopts an array type high voltage bare electrode and inclined split type thick wall pipe body and conducts the second stage of array type high voltage electric field crude oil deep dehydration process, and residual water is separated out. A gas-liquid separator removes condensate oil small oil drops in the wet gas according to the two-stage filter liquid effect. A mixer fully stirs the water-contained crude oil and chemical agents through a mixer wheel to form crude oil emulsion. Remote automatic control over all stages of underwater on-line separation operation is achieved and safe flowing of the operation is guaranteed through an underwater on-line control system.