3-phase AC deep sea motors are widely used on many deep sea equipments.These motors usually get their power source via umbilical cables from the surface vessel.Because of their low power factors,their umbilical cables must transmit a lot of reactive power,resulting in unnecessary high currents.This may lead to many problems such as high temperature of the cables,big voltage fluctuations,etc.A good solution to this is to compensate the reactive power in situ.This paper takes the deep sea rock drill as an example,brings about a common calculation method and an overall technology for in-situ reactive power compensation of deep sea motors,and gives the lab verification experiment results.