The influence of welding parameters on weld depth and porosity is investigated in partial penetration fiber laser welding using 7 kW fiber laser, and the relationship between weld and keyhole depthes is discussed. The suppression of porosity is also investigated using triangular wave power modulation. The results indicate that the keyhole depth coincides approximately with the weld depth in laser welding. The weld depth and the porosity reduce with the welding speed increasing. When the defocus distance is 0, the weld depth and porosity ratio reach the maximums, whereas both of them decrease as the laser focus position is off the sample surface. In a wide range of pulse frequency (20~125 Hz), the porosity is effectively suppressed by triangular wave power modulation for fiber laser welding, and the optimal frequency is 60 Hz. The keyhole behavior is analyzed by X-ray transmission imaging system to find that the stabilization of keyhole is promoted by power modulation, which results in the suppression of porosity.