I could renamed the field of timechart. For example: Changed count to 'YYY' .
But,I couldn't renamed the '_time' field,Splunk told me"Error in 'timechart' command: The specifier '_time' is invalid."
Can you tell me how to change the '_time' field?
I think you're referring to the fact that in timechart reports, the x-axis title is often "_time" unless it has been changed to something else.
Go back and use report builder again to build and save your timechart, in other words click on 'build report' or 'show report' from the search UI, then on the 'formatting' page of report builder, you'll see little links that will give you 'X-axis' and 'Y-axis' options.
Click each of those and they will allow you to change several things including the x-axis title and y-axis title. In timecharts the x-axis has a default axis title of _time but it can be changed there and when you save the report it will be persisted.