Hi @tianchi
Kindly see these instruction to import your csv https://support.atlassian.com/jira-cloud-administration/docs/import-data-from-a-csv-file/
Especially:
Creating sub-tasks
You can create sub-tasks of issues by:
Adding two columns to your CSV file with the headings "Issue Id" and "Parent Id"
Ensuring each regular (non sub-task) issue is given a unique (sequential) number in the "Issue Id" column. Do not include any value in the "Parent Id" fields for regular issues.
Referencing the unique "Issue Id" number of the regular issue in the "Parent Id" column. Do not include any value in the "Issue Id" fields for sub-tasks.
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