The battery is easily drain even i replaced it an new battery, what is the posible problem of my vespa lx 150 ie? Ty

Many answers you need a multimeter or ammeter to measure the quiescent current drawn (power used when nothing turned on)

Set it on the 10a scale and measure the current drawn ,put the leads in series with the battery feed wire.

Causes could be a faulty regulator pack or a short on the stator of the generator,a scuffed cable shorting to earth ,(usually around the brake light switch) or a faulty switch causing a lamp to light continuously . I could go on but that should give you a starting point.
good luck