Rinse the bean sprouts thoroughly, blanch in boiling water for 10 seconds, then rinse under cold water and drain.
Slice the onion and carrot thinly.
In a bowl, mix vinegar, sugar, salt, and water to make the pickling liquid.
Place the prepared vegetables in a sterilized container, pour over the pickling liquid, and let it marinate in the refrigerator for at least 2 hours.