Hello Zhaoja1,

Dealing with differences in balances can be a strenuous experience but don't worry, I'm here to help. QuickBooks provides you with the necessary tools and resources that help you with keeping your transactions in order. 

Based on what you've described, I encourage you to check the transactions included in GST your report. Here's how: 

1. Select Taxes from the menu on the left and click Sales Tax at the top.
2. Go to the Action column on the right and click the black drop-down arrow next to Prepare return.
3. Select View summary.
4. Click the amount under the Total column for Line 101 Sales and other revenue. This will pull a Detail report of all the transactions that make up line 101 on the return.

You can compare this Detail report to your income reports to see which transactions are causing the difference.

Another tool that can help out here is the Transactions without sales tax report. Here's how to access it:

1. Go to the Sales Tax page.
2. On the right-hand side, click the View Reports dropdown.
3. Select Transactions without sales tax.
4. If needed, adjust the date range for the report and click Run report.

This will show you all transactions for the time period that either doesn't have a sales tax code or are marked Out of Scope. Give this a try and if you have questions, feel free to reach back to me in the comments below. 

That's good news! Great job on finding the discrepancy. :)