She's just letting off steam．

她只是发泄一下而已。

　　let off steam，字面意思是让蒸汽蒸发，引申为“发泄强烈的感情，发脾气”等。
例句：Don't take what she says too much to heart，she's just letting off steam．
　　别把她的话往心里去，她只是发泄一下而已。