Abstract: Investigated the effect of hydrogen environment on the mechanical properties, fatigue crack growth rate, fracture toughness, and hydrogen embrittlement mechanism of L245 pipeline steel. The results show that with the increase of hydrogen concentration in the environment, the elongation and reduction of area of L245 steel decrease significantly, the hydrogen embrittlement sensitivity increases, while the tensile strength and yield strength remained almost unchanged. Hydrogen environment significantly reduces the crack growth threshold of the steel and increases the sensitivity of crack growth rate to the stress intensity factor range. The increase in hydrogen concentration gradually reduces the fracture toughness of the steel and increases its brittleness. The microscopic crack growth patterns showed that cracks in the non-hydrogen-charged specimens mainly exhibit transgranular cracking, while those in the hydrogen-charged specimens showe a mixture of transgranular and intergranular crac-king, with the crack growth direction at approximately 90° to the tensile direction. The analysis of hydrogen embrittlement mechanism shows that hydrogen molecules first physically adsorb onto the steel surface and dissociate into hydrogen atoms under the catalysis of iron atoms, which then diffuse into the steel matrix in an interstitial manner. These hydrogen atoms tend to aggregate at ferrite grains or segregate at grain boundaries and dislocations, changing the crack growth mode from single transgranular cracking to a mixture of transgranular and intergranular cracking. At low hydrogen concentration (less than 10%), cracks mainly propagate within ferrite grains, showing transgranular fracture.At high hydrogen concentration (greater than 20%), hydrogen atoms significantly segregate at grain boundaries, weakening the grain boundary strength and increasing the proportion of cracks propagating along grain boundaries.