The payment base of social security is calculated according to the average salary of the previous year, so if I changed companies last year, how can I calculate the payment base this year?

The payment base of unit social security is based on the first month of your employment in this unit or the average salary of this unit last year.

Last year's average salary was my own in the unit, not counting those who left the unit before.

If you joined last year, the average salary will be calculated according to the working months in the unit time. Employment this year is still based on the first month.