As against the defects of the conventional calculating method for the straightening forces of the roller type straightening mills and the difficulty of straightening forces monitoring of all rollers directly, this paper, through the straightening forces engineering monitoring of the heavy rail-beam straightening mill 1300 (the first and biggest set in China) and straightening forces waveform analysis of top rollers, analyzed straightening forces and straightening process by way of pressure steps (straightening forces component). The built-up process of the indeterminate straightening forces system and the relations among straightening force of every roller was revealed. The division unit measurement theory of straightening forces system and straightening force componentit was discribed and the altogether convenient engineering monitor measurement method was introduced. The general straightening force equation and its engineering computation was given. The straightening force nomogram of H-beam 56 and rail 50 was provided.The above-mentioned theory and methods of straightening forces may be used for heavy roller type straightening mills.