Shanhaiguan has a sharp continental climate moderated by the sea. Summer (June–August) is warm (25–30°C) and the only time when the beaches at Old Dragon's Head are usable; it is also peak Chinese domestic-tourism season, with crowds. Autumn (September–October) is the best time to visit — warm dry days, cool nights, low humidity, the mountains turning yellow and red, and the early-October national holiday week aside, manageable crowds.
Winter (November–March) is cold (-5 to 5°C) but very clear, with frequent dustings of snow on the wall — the most evocative season for photographs. Spring (April–May) is short and prone to spring dust storms blowing in from Mongolia; check the air-quality forecast before planning a Jiao Mountain hike.
The pass is busiest during Chinese national holidays (Spring Festival in January/February, Labour Day in early May, National Day from October 1–7). Avoid those weeks if you can.