This paper presents a VLSI architecture design of unified motion vector (MV) and boundary strength (BS) parameter decoder (PDec) for 8K UHDTV HEVC decoder. PDec in HEVC is deemed as a highly algorithm-irregular module, which is also challenged by high throughput requirement for UHDTV. To solve these problems, four schemes are proposed. Firstly, the work unifies MV and BS parameter decoders to share on-chip memory and simplify the control logic. Secondly, we propose the CU-adaptive pipeline scheme to efficiently reduce the implementation complexity. Thirdly, on-chip memory is organized to meet the high throughput requirement for spatial neighboring fetching. Finally, optimizations on irregular MV algorithm are adopted for 43.2k area reduction. In 90nm process, our design costs 93.3k logic gates with 23.0kB line buffer. The proposed architecture can support 7680x4320@60fps realtime decoding at 249MHz in the worst case.