Experimental Group: Conventional treatment plus 3 times of MSCs(4.0x107 cells per time) intravenously at Day 0, Day 3, Day 6.Control Group: Saline containing 1% Human serum albumin (solution of MSC) 3 times of placebo (intravenously at Day 0, Day 3, Day 6)Other: Intravenous saline injection (Placebo) Intravenous injection of 3ml of 0.9% saline on day 1, day 4 and day 7, based on routine treatment of COVID-19. Here, ALB is linked to COVID-19.