The present invention is directed to a luminescent immunoassay method for detecting an analyte in a liquid sample with high sensitivity. The invention provides a unique combination of (i) using a probe having a small sensing surface area for binding analyte molecules, (ii) using a high molecular weight branched polymer conjugated with multiple binding molecules and multiple luminescent labels, and (iii) cycling the probe having immunocomplex formed back to the reagent vessel and amplification vessel 1-10 times and repeating the reaction with the reagent and the amplification polymer, to improve the sensitivity of detection level. For each cycling, the luminescent signal is increased significantly over the noise. The present invention is also directed to an electrochemiluminescent detection system for measuring a chemiluminescent signal on a probe tip.