In order to understand in depth plasma behavior during ultra-high power fiber laser deep penetration welding, the plasma inside and outside the keyhole is observed, and the spectrum of fiber laser-induced plasma is measured and analyzed. Based on the measured data of plasma, the electron temperature and electron density, ionization degree and pressure are calculated, and the characteristics of plasma parameters at different values of keyhole depth and outside the keyhole are investigated. The results indicate that the distribution of plasma inside the keyhole is uneven, and the vapor plume is much bigger outside the keyhole. The spectrum of plasma show that the fiber laser-induced plasma is weakly ionized and radiates a few spectral lines. The further calculation results also confirm that the plasma induced by fiber laser is in a weakly ionized state. However, the electron density of plasma still stays in a high level, and the transient pressure of plasma is up to hundreds of times as large as atmospheric pressure.