This study applied hydrogen assisting combustion system to the 2 t/hr once through steam boiler using heavy oil as fuel. The hydrogen generator that is methanol reformer is integrated with the boiler. The methanol reformer can generate hydrogen mixture gas (H_2+CO_2). The hydrogen mixture gas was injected into combustion chamber of the boiler using hydrogen lance to proceed hydrogen assisting combustion. For performance testing, we compared the concentration variations of NOx and CO in the flue gas before and after using hydrogen assisting combustion. Meanwhile, the long term running test was performed to analyze the benefits of energy saving and CO_2 reduction. So far the boiler have run 291 days after using hydrogen assisting combustion system and no abnormal problem occurred. Compared with baseline test results, the average water-oil ratio of boiler increased 6.03% during this period of running.