There is no walk-up sale — you reserve on the museum's own channels
The museum's ticketing page states that all visitors, including Hong Kong, Macao and Taiwan compatriots and foreign visitors, must reserve and pre-purchase tickets through the official website or the official WeChat public account. No on-the-day ticket window is offered on that page, so a plan that assumes you can buy at the gate has no fallback.
Official source: Emperor Qinshihuang's Mausoleum Site Museum (bmy.com.cn) — Ticketing and visiting rules (source dated standing ticketing-rules page; read August 26, 2026)