The invention relates to the field of coal gasification industry, and particularly relates to a treatment method of black water generated by coal gasification and a coal gasification method as well asa system thereof. The treatment method of black water comprises the following steps: performing swirl solid removing treatment on the black water; performing high pressure flash distillation, low pressure flash distillation and vacuum flash distillation on the black water obtained after the swirl solid removing treatment sequentially; performing a contact reaction on the black water obtained after the vacuum flash distillation with calcium hydroxide; performing flocculation treatment on the black water obtained after the contact reaction under the existence of a flocculating agent; performingaeration treatment on the black water obtained after the flocculation treatment by introducing CO2; and performing sedimentation on the black water obtained after the aeration treatment, and performing microfiltration treatment, ultrafiltration treatment and reverse osmosis treatment on the supernatant obtained from the sedimentation sequentially. According to the method, hardness descending anddesalination can be effectively realized, and when the treatment method is used in a coal gasification technology, not only can the problems of scaling, blocking and corrosion caused by ash water be solved, but also recovery rate of water can be improved under the conditions of only increasing part treatment equipment without influencing the technology and layout.