其實,at the party 或 on the party都可以,具體怎么用,要看語境.基本上沒有in the party（除非party是政黨的意思,但題目已經(jīng)指明是派對）
舉例：
Here is a rough guide to show you how many varieties of food and drink you will need at the party,
這里有個指南,告訴你如何為派對準備多種食品和飲料.
On the party, the host announced their engagement.
在派對上,主持人宣布了他們訂婚的消息.
John and Mary first met each other on a friend’s birthday party.
約翰和瑪麗最初在朋友的生日派對上認識.
或者大致上這么說吧,談論到人,用on the party,談論到事情,at the party.On the party其實用得頗多,但具體使用時仍須看語境和具體要說什么.