You can literally use "frog in the well". It's one of the few cases where the Chinese Proverb is the same as the English saying.

"Narrow minded" or "ignorant"

I am a 68-year-old American English teacher and I disagree with both answers above, especially Donald's. There is no English idiom about a 'frog in a well', and just a bit of research will confirm that it is from a Chinese fable. You could not use it here and be readily understood at all. I further disagree with with Mahdiyya's English equivalents, at least as I understand the meaning of the Chinese idiom. The 'frog' is not narrow-minded or ignorant by choice, but rather uninformed by circumstance, having never had the opportunity to experience a wider view of all that surrounds him. Those terms are too harshly judgmental. A person who has led a sheltered life has an innocent view of the world, unaware of its evils. A possible English idiom with similar meaning would be "a babe in the woods" -- someone who has not had much life experience and trusts other people too easily.

很难相对应地翻译