I am using Ubuntu for a quite long time now. I am concern about laptop getting hot. So i would like to know is it safe to have ACPI temperature 90 C ? Or it could damage my laptop ?
ACPI temperature is most likely your motherboard temperature - not the CPU. Mobile CPUs temp limit is usually little above 100C - depending on CPU type. Example i5 spec - see Tjuntion temperature. I would say that 90C ACPI temp is bit too high unless under heavy load like gaming. If the temp is at the level all the time I would suggest getting your laptop serviced or cleanning it up by yourself if you know how.
It's not dangerous really as like others have said the PC will automatically power off if it gets too hot. Just make sure to backup and save regularly and maybe think about cleaning the fan. Another thing is to check your HDD life using one of the many tools out there as when I had this problem of high temperatures and overheating it turned out the HDD was near the end of it's life and this puts more strain on other components.