Question: How to make a 2d animation of a person pulling a sword strapped to it's back

https://www.videocopilot.net/tutorials/light_sabers_v2/

just like this?use mask can do it

You need to separate the footage into layers. The most efficient way to do that depends entirely on the footage. A screenshot would help. 

Unless you shot the footage specifically for keying with a solid color background like a green screen, the process is called rotoscoping.  After Effects has a tool that will automate that process a bit called Rotobrush but it will only work well if you shot your footage with Rotobrush in mind or are extremely lucky. The background must be a different color than the foreground elements, the edges must not have too much motion blur, and there can't be too much detail in the background.

If you have to do it manually then motion stabilizing the shot first may help. Generally, I'll load up a solid layer with a similar color to the foreground I'm trying to roto, set the blend mode to Overlay and then start animating a mask. Sometimes you need more than one of these layers and for most action involving people, you need more than one mask. Once you get all of your masks animated you can set them as track mattes for copies of your footage and your sword layer using Set Matte or by simply putting the track matte layer directly above the layer you want to mask and setting the appropriate track matte setting in the timeline.

All of this requires a fairly decent knowledge of how layers work, how masks and blend modes work, how to set keyframes and how to navigate around the timeline. I strongly suggest you start with the basic tutorials available in the User Guide if you are new to After Effects. You'll find a link on this forum, on the AE home screen (Learn), and on the AE Product page. I'll even post the link for you here: Learn AE

Probably the best tutorial I can point you to that explains the principals of rotoscoping is from Scott Squires. He is a visual effects master. The tools he is using in this series of tutorials are not Adobe products, but the technique and the theory is exactly the same. Layers, multiple masks, planning and lots of work. Take a look: Effects Corner: Rotoscoping - Part 1 and Effects Corner: Rotoscoping - Part 2. If you really want to know how this stuff works go through both parts. 

You might also want to check out his YouTube Channel: https://www.youtube.com/channel/UCzTfI6gSaLlLsqKUbv-TN6A

Using alpha matte might be what you need to pull off that effect. 

Below's a short (around 2 minutes) but really helpful tutorial. This is really easy and will be useful on lots of projects. 

https://www.youtube.com/watch?v=vEfT4G56WHs