In this article, we experimentally studied combustion characteristics of a direct-injection spark-ignited engine fueled with natural gas-hydrogen blends. For a specific operation mode, the results show that the heat release rate decreases with the increase of hydrogen fraction in the blends when hydrogen fraction is less than a certain volumetric fraction while the heat release rate increases with the increase of hydrogen fraction in the blends when hydrogen fraction is over a certain value. This phenomenon indicates that only when the hydrogen fraction in natural gas reaches a certain fraction can a large improvement in combustion be realized. Flame development duration, rapid combustion duration, and total combustion duration increase with the increase of hydrogen fraction in the blends when hydrogen fraction is less than a certain volumetric fraction, while they decrease with the increase of hydrogen fraction when hydrogen fraction is over the value. The crank angle of the center of heat release curve moves away from the top-dead-center with the increase of hydrogen fraction in the blends when the hydrogen fraction is less than a certain volumetric fraction, and it moves close to the top-dead-center when hydrogen fraction is over the certain value. Maximum cylinder gas pressure, maximum mean gas temperature, maximum rate of pressure rise, and maximum heat release rate decrease with the increase of hydrogen fraction when the hydrogen fraction is less than a certain volumetric fraction, and they increase with the increase of hydrogen fraction when hydrogen fraction is over the certain value. For fixed injection duration, the influence of hydrogen addition on natural gas-hydrogen mixture combustion is larger at low engine speed operation condition than that at high engine speed operation condition.