Transmission towers are typically composed of angle steel members connected by ordinary bolts to form spatial truss systems, in which joint slip under axial loading can significantly influence structural performance. In subsidence areas, corrective lifting of tilted towers may cause internal force redistribution, transforming some compression members into tension members and resulting in joints subjected to both compressive and tensile forces. To investigate the slip deformation behavior of angle steel bolted connections under bidirectional axial loading, a series of experiments was conducted on specimens with different angle sizes and bolt numbers, complemented by finite element analysis. The results show that the load–slip relationship exhibits distinct staged characteristics, which can be divided into an initial linear stage, a slip stage, and a hole-bearing stage. The initial slip displacement is generally less than 1 mm, while the slip load and ultimate capacity increase significantly with bolt number, with the ultimate capacity under tension increasing by up to approximately 160% as the number of bolts increases from one to three. Although the slip evolution under compression and tension is generally similar, pronounced differences appear near the ultimate state, indicating a clear directional asymmetry. Based on these findings, a three-stage piecewise mechanical model is established, and a simplified bidirectional slip model is proposed by introducing asymmetric ultimate displacement and capacity parameters. Finite element simulations reproduce the failure modes and load–slip responses with good agreement, confirming the validity of the proposed model. The findings provide a useful reference for the design and performance evaluation of angle steel bolted connections in transmission tower structures.