Abstract: Evaluating the transfer ability of trans-regional power system channels by the tie-line available transfer capacity (ATC) may result in an unfeasible tie-line plan under the sub-region operation constraints or lead to extra re-dispatch costs. This paper proposes an analysis method of the tie-line power feasible region based on the combination of the distant and the adjacent time intervals, which is adapted to the demand of the day-ahead dispatch scenario. Firstly, based on the polytope projection theory, the impacts of time-coupling relations on the feasible region calculation and countermeasures are analyzed. Then, in accordance with the characteristic of time intervals coupling in the day-ahead dispatch, the strategy of the distant and adjacent time intervals combination is designed and the feasible region models are established. In this way, the impacts of the coupling constraints of both the distant and adjacent time intervals are considered. And the tie-line capacity of each time interval in the day-ahead dispatch is depicted by the combination of the feasible region calculation results of the distant and adjacent time intervals. Finally, based on the 118 nodes system the different calculation results of feasible regions by several methods are compared, and the complementary of the distant and adjacent time intervals combination strategy is analyzed. Based on the 39 nodes system interconnected with the 118 nodes system, the influence of the tie-line capacity feasible region on the sub-region dispatch plans is analyzed, which verifies the efficiency of proposed method.