The present invention relates to an ultrasensitive rapid antigen detection kit for detecting N protein from SARS-CoV-2 virus in a sample, and methods for detecting N protein. The kit includes a sample collector, a collection and enrichment tube and a rapid antigen detection component with a test strip. The collection and enrichment tube contains a first part containing lyophilized ATPS reagents and a second part containing a lysis buffer. The sample is mixed with the lyophilized ATPS reagents and the lysis buffer to form a mixture. The enrichment is realized by concentrating the target analyte into one of the two phases formed by ATPS.