A novel cascade fluorescence signal amplification strategy based on the rolling circle amplification (RCA)-aided assembly of fluorescent DNA nanotags as fluorescent labels and multiplex binding of the biotin-streptavidin system was proposed for detection of protein target at ultralow concentration. In the strategy， fluorescent DNA nanotags are prepared relying on intercalating dye arrays assembled on nanostructured DNA templates by intercalation between base pairs. The RCA product containing tandem-repeat sequences could serve as an excellent template for periodic assembly of fluorescent DNA nanotags， which were presented per protein recognition event to numerous fluorescent DNA nanotags for assay readout. Both the RCA and the multiplex binding system showed remarkable amplification efficiency， very little nonspecific adsorption， and low background signal. Using human IgG as a model protein， the designed strategy was successfully demonstrated for the ultrasensitive detection of protein target. The results revealed that the strategy exhibited a dynamic response to human IgG over a three-decade concentration range from 1.0 pM to 1.0 fM with a limit of detection as low as 0.9 fM. By comparison with the assay of multiple labeling antibodies with the dye/DNA conjugate， the limit of detection was improved by 4 orders. The designed signal amplification strategy would hold great promise as a powerful tool to be applied for the ultrasensitive detection of target protein in immunoassay.