Concerning the low thermal efficiency and high emissions of spark ignition (SI) engines, an experimental study was performed aiming at improving SI engine economic and emissions performance under a typical city driving condition of 1500 rpm and at the stoichiometric equivalence ratio with four intake manifold absolute pressures (MAP) of 39.3, 48.1, 56.4, and 67.8 kPa and four hydrogen volumetric fractions of 1, 1.5, 2, and 3% on a modified 4-cylinder gasoline-fueled SI engine on which hydrogen can be injected into the intake ports sequentially via a self-developed electronic control unit (DECU) and mixed with gasoline online. The DECU can be used to adjust the injection timings and durations of hydrogen and gasoline to accomplish the specified volumetric fractions of hydrogen in air and the mixture equivalence ratios. The engine test results demonstrated that, compared with the original gasoline engine, the brake thermal efficiency and the cycle-by-cycle variation of the peak in-cylinder pressure were averagely increased from 25.12 to 28.35% and reduced from 6.8 to 3.62%, respectively, at the hydrogen addition fraction of 3% and four specified MAPs. Both ignition delay and the rapid burning duration were shortened; the peak in-cylinder pressure and temperature were increased; CO2 and HC emissions were obviously lower with the increasing fraction of hydrogen in the intake. However, NOx and CO emissions were also raised. In a word, hydrogen addition is a potentially applicable measure to improve SI engine performance.