I have a baby 7 month 7 days, i don't know why he always cry at night during sleeping?

Probably separation : As most infants reach 6-9 mo they become aware of how important their primary caregivers are to their happiness & well being. At that point their brain doesn't usually let them understand that people they don't see are still around. If baby is awake as you check them at night and settles immediately when they see you this is likely. Some babies just fuss as they drift thru sleep stages to settle.

Babies and sleep: The reason can vary but common causes for babies to cry during sleep include night terrors, teething, habit, pain such as undiagnosed ear infection, possibly nightmares, and heartburn.

Sleep: Children could cry due to so many reasons he is an infant and needs lot of sleep, make sure he is fed at timely intervals and has burped before sleeping, rule out other causes such as colic etc.