This study investigates the stability of lifting operation for a mobile crane. First of all, in
accordance with international standards and national regulations to calculate stability grades basing
on the tipping line composed by two supporting points of the crane. Proposed two extend topics,
one of which is simultaneous occurrence of wind blow and crane rotate on a slope, estimation of
rotating speed limitation for a boom should base on the linear superposition principle of combined
load operating security equations. The other will be the danger of lifting submerged stuff. The third
issue discussed in this article is the condition of losing anyone of four supporting points. In fact, this
condition should be taken as the critical stability. Because the two adjacent supporting points beside
the losing reaction point could create a tipping line, this means that the stability moment equal to
overturning moment. Due to the complex structure of mobile cranes, there are various changes in
postures at simulating of crane lifting operation. It must apply computer-aided modeling to obtain the
distribution of weight under various postures of crane to facilitate analysis. This study uses extended
legs to be supports, and applies the load of boom with the reaction force of opposite side supporting
legs at zero as overturning condition. Utilizing the global lifting simulation to calculate the overturning
load, the equal load line of crane lifting operation will be obtained and according to deduce primary
and secondary dangerous zone.