Many task on one day

in our system, one day we consider it a task in a day there can be two or three tasks. I want to change the start time of the second task when the first task’s end time will be delayed. I have no way to do that

Hello Linh,
You can connect split child tasks with a link and use the autoscheduling feature to move the successor task to a new position after moving the predecessor task.
Also, you may want to consider adding the options below, so that the tasks can be placed anywhere:

gantt.config.duration_unit = 'hour';
gantt.config.round_dnd_dates = false;

In the following snippet, you can move Stage #1 to a new place, and Stage #2 will follow that task: