The invention relates to a low-pressure discharge plasma water treating apparatus, mainly comprising a discharge power supply, a high voltage electrode, a reactor, an atomization sprinkler, a water inlet pipe, a water outlet pipe, a water flow monitor, an air exhaust pipe, an air valve, a vacuum pump, a sewage pool and a clean water reservoir. According to the invention, without a water pump, a low pressure environment in the reactor can be maintained via only one vacuum pump under the condition that water flows through the reactor; electro-discharge is carried out in the low pressure environment, and large-volume, high-activity and non-thermal equilibrium plasma is generated; the treating apparatus can effectively remove pollution sources like heavy metal ions, organic matters and microbes in water, has high treatment efficiency, is generally applicable to sewages having different pollution degrees and produced by different industries and provides activated water for special industries. The invention further relates to a plasma water treating method which realizes low pressure discharge under the condition that a water cycle between the interior of the reaction chamber and the outside is maintained without usage of a forced exhaust type water pump. With the method, system energy consumption is greatly reduced.