English Translation
What surprised me was his cold attitude.
Unnatural word-for-word translation. It’s better to swap the subject and the object when you translate in Japanese.
彼の冷たい態度には驚いた。or
彼の冷たい態度には驚かされた。
You can also omit は from my proposed sentence. But inserting は captures the nuance of the English structure “what surprised me was…” Inserting は suggests that “what made me surprised was NOTHING ELSE BUT his cold attitude”. 助詞「は」is often used when highlighting the difference between two (or more than two) things/situations.