The weak AC tie-line power fluctuation caused by DC block is a realistic problem affecting the safety of UHV AC/DC power grid in China. With more and more DC projects put into operation, the importance of the study on power fluctuation influence mechanism of weak AC tie-line caused by different DC block is recognized. Taking Xitai DC block and Jinnan DC block in North China Power Grid as the typical study case, weak tie-line power fluctuation mechanism is analyzed through decomposing the tie-line power variation into inter-area oscillation mode component and inter-provincial oscillation mode component. Based on theoretical derivation and simulation analysis, it is found that the peak value of inter-area oscillation mode component of tie-line power increases with the increment of electrical distance from faulty DC location to tie-line location. Moreover, important principle of inter-provincial oscillation mode component of tie-line power is obtained by using generalized phasor method. The phase variable quantity of the inter-provincial mode component of tie-line power with the change of faulty DC location is equal to the phase variable quantity of strongly correlated generator defined by power system small signal analysis. The locations of Xitai DC and Jinnan DC are very special, which are near the relative oscillation generator groups concerned with inter-provincial oscillation mode. Thus, the phases of the inter-provincial mode component of tie-line power are opposite to each other for different DC block, which leads to the change of tie-line power peak value and occurrence time. This work is supported by Science and Technology of State Grid Corporation of China “Research and Application Demonstration of Aggregation Effect of Multi-point Layout Distributed Energy Storage System in Power Grid”.