思わず吹き出してしまった

couldn't help but burst out laughing

→ 思わず
phrase Rare #39,600
couldn't help but burst out laughing Describes unintentionally bursting into laughter, often in response to something funny or amusing. しまった adds a sense that it happened despite the speaker's intentions, sometimes with mild embarrassment or regret.
JA: その写真を見て、思わず吹き出してしまった。
EN: I couldn't help but burst out laughing when I saw that photo.

思わず吹き出してしまった

I couldn't help but burst out laughing.

彼の髪型を見て思わず吹き出してしまった

I couldn't help but burst out laughing when I saw his hairstyle.

私は思わず吹き出してしまった

I couldn't help but burst out laughing.

その光景を見て私は思わず吹き出してしまった

When I saw the sight, I couldn't help but burst out laughing.