Constellations are calculated according to the solar calendar, but they are different every year. How can it be accurate?

The constellation is determined by the date of the solar calendar on the day you were born. Your birthday every year is a lunar birthday, so the Gregorian calendar date changes every year. You don't have to care about the Gregorian calendar date of your birthday every year. As long as you remember the Gregorian calendar date of your birth, that constellation is accurate.