Hello @YuZhang,
First of all your data at this point looks pretty normal. 
Let's walk through these visualization to get a better idea of what they are telling you. Sound good?
demux.qzv
When looking at the interactive quality plot, you can analysis the quality of the data. "Good Data" is really hard to quantify but a good rule of thumb is anything around 30 is a pretty good quality score. Why do you want to truncate, in part it is so that dada2 doesn't get lost in the weeds of a lot of not good quality data. If your case there is really no "bad data" (This would be where you see a fall off in the quality of data. You can see this here) so you can truncate at the very end of your sequences length. If you wanted you could truncate at 210 but 200 works just fine! :qiime2:
stats-dada2-3.qzv
This is an overview of the quality control that dada2 did to your data. The short and sweet of it is you don't want to loose too many sequences at one step in the quality control. The steps being the columns at the top. When looking at your data here nothing is out of the ordinary.
rep-seq.qzv
The rep-seq files shows all your features and their sequences. The range you have of sequence lengths seems good to me like I said previously. Most of your data is around 400 in length and looks good.
I know that was lot! I hope that this helps not only with this dataset but in future analysis as well! :qiime2:
Chloe 