The hydraulic oil tank of a 3boom rockdrilling jumbo needs to be reformed. Calculations should be made to verify whether the capacity of the hydraulic oil tank meets relevant requirements and whether the heat balance of the hydraulic system is rational, so as to ensure that the reformed hydraulic oil tank can meet the operation requirements of the rockdrilling jumbo. In the paper, the capacity of the hydraulic oil tank is calculated in terms of the displacement of the hydraulic pump, the quantity of the oil reserved in the hydraulic cylinders and pipes and the variation of the oil level of the hydraulic oil tank during operation, which shows that the reformed hydraulic oil tank can meet the operation requirements; the heat emission power of the actuating unit of the hydraulic system, the heat emission power of the hydraulic oil tank and the heat emission power of the water cooling system are calculated and solutions are proposed for the heat emission problem. The reformed hydraulic rockdrilling jumbo has passed the workshop test and the field test, which proves that the theoretical calculation made is rational. The paper can provide reference for the reform of other equipment in the future.