1,what's record cached in the table cache_form?
2,my site's cache_form table is too big(1.0G). the cron hasn't been working for a month. when i run it manually, it still can't work. why. the drupal version is 6.19.
A hot fix, would be to manually delete all the rows from the database. However, the problem is not that form table it getting to big, but that cron is failing. Not running cron on your site will cause a lot of problems, in the future if you don't find and fix that problem.