@Yanfei-Geng, I don’t think the data from alpha-rarefaction is exactly what you’re looking for. Does the following get you what you need? The table.qza file that I’m starting with is available here.
$ qiime tools export table.qza --output-dir ./
$ biom convert -i feature-table.biom -o feature-table.tsv --to-tsv
$ biom head -i feature-table.tsv
# Constructed from biom file
#OTU ID L1S105 L1S140 L1S208 L1S257 L1S281
4b5eeb300368260019c1fbc7a3c718fc 2222.0 0.0 0.0 0.0 0.0
fe30ff0f71a38a39cf1717ec2be3a2fc 5.0 0.0 0.0 0.0 0.0
d29fe3c70564fc0f69f2c03e0d1e5561 0.0 0.0 0.0 0.0 0.0
868528ca947bc57b69ffdf83e6b73bae 0.0 2276.0 2156.0 1205.0 1772.0
154709e160e8cada6bfb21115acc80f5 812.0 1176.0 713.0 407.0 242.0
Here, the first column contains feature or OTU ids (e.g., 4b5eeb300368260019c1fbc7a3c718fc), the first row contains sample ids (e.g., L1S105), and the values in the table indicate the number of times each feature was observed in each sample. So, in this case, feature 4b5eeb300368260019c1fbc7a3c718fc was observed 2222 times in sample L1S105, and zero times in each of the other samples.