I assumed the baby(OPP roughly 3weeks)was killed by the mother because when I realise the body ,it only has its lower body left,and the upper body was gone.It was born twins ,the other baby is still alive but very skinny .should I separate the other baby before the mother kills it again?

Tough call. She may have culled one joey so the 2nd would have a better chance of survival. She may not have had enough milk or energy for both. If she is underweight and not able to properly care for the remaining g joeys the same thing may happen. Others will have better advice u may want to pick up a joey kit incase the mother stops caring for him.

So it means if she killed one she will not kill the other baby?I have been observing .but it's hard to decide.because the mother will sometimes leave this baby alone,and also sometimes bring the baby along .i do not want another baby to get killed.

Really difficult, there are many reasons why a mother can kill her joeys, they can sense the joey is sick, they can lack milk to feed both so choose to reject one, and so on.

If she is alone to raise the joeys, it's more difficult for her, the father would normally take care of the joeys when she is out to eat and exercise, which can explain why she sometimes leaves the joey alone and sometimes brings him along.
One thing is., does she come to check when the joey is crying? If you don't hear him crying at all, or don't see him trying to crawl out of the sleepåing pouch, he is likely fed.

The fact that he seems skinny might mean that she was lacking milk to feed both of them properly. In any way, removing him from his mother is not necessarily a good idea, unless you observe clear signs of rejections. What you can do however is check often, and if he keeps being underweight, increasing the mother's protein intake, and maybe supplement the joey by adding some hand feeding with wombarro milk replacer or similar products.

How about so you all think this is a good idea ? I will bring the baby along to work in the day (take care of the baby and feed milk to her every hour) and then in the evening ,I will put the baby back to the mother(at least I can keep an eye of the mother to prevent her killing the baby)

I think it would stress the mother even more and make her more likely to just reject the joey totally alltogether

It is not a good idea to separate them temporarily, the mom might not accept the baby back again.

Thanks a lot! Can you teach me how to hand raise a oop 3weeks joey? As I feel it is really unsafe to put the joey back again

You can find hepl for rejected joeys here:

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Sorry. Look for Suz's sugar gliders, help for rejected joeys.