Abstract: The toughness weak points of multi-layer and multi-pass weld of 30Cr2Ni4 MoV refractory steel steam turbine welded rotor were studied by means of simulated heat welded layers with the emphasis on the forming of the toughness weak points and its influence on the toughness. The methods of optical microscope analysis, scanning electron microscopy analysis and transmission electron microscope analysis were utilized. The experimental results show that there are many M-A constituents in the carbon-rich areas of welded layers, which is disadvantageous to the toughness. The influence of the M-A constituents on the weld toughness is closely related to the distribution, pattern and dimension. M-A constituents in the refined zone of welded layers decompose at second peak temperature of 680℃, large particle M-A constituents at prior austenite grain boundaries and bulk grain M-A constituents in prior austenite grains are found in the incomplete phase change zone of welded layers at second peak temperature of 820℃, and M-A constituents distribute uniformity at 1 050℃ with the disappear of prior austenite grain boundaries.