For that very reason Steam gives you a bunch of 1 time login backup codes when you setup your Steam Guard and tells you to write them down. Guess you did not do that?
If you dont have access to that phone anymore you will need to contact Steam Support and go through the account recovery process, providing evidence that you are the account owner.

Help! I sold or lost my phone and I can't log in anymore! What do I do?
If possible, you should contact your wireless service provider and get a replacement phone with the same phone number. Steam will be able to send SMS messages to your replacement phone, and you will be able to recover your account as described below.

If you no longer have access to your phone and don't have your recovery code, you'll need the assistance of Steam Support. Use the "Help me with my issue" button at the top right of this page to get help removing your authenticator. If you don’t have access to the email address that is currently associated with your Steam Account please provide proof of account ownership within your ticket.

引用自 LinTach：

I can't log into my account on phone and website, it says that i need to use my steam guard, which is on my broken phone, i don't know what to do

Put the SIM into a new phone and migrate the authenticator via SMS.