The invention discloses an ultrasonic detection method of the solid layer thickness h in a microcellular foaming injection molding product. The method includes: (1) for an online injection molding system, a series of ultrasonic waves are emitted to the product in a molding process in a direction vertical to the flow direction of a plastic melt, and echo information of the series of ultrasonic waves is acquired; (2) after the plastic melt flows to a detection area, the ultrasonic waves are reflected from a contact surface of a template and a polymer, the maximum amplitude Amax of the echo information is received and analyzed (the maximum amplitude usually occurs at previous echo waves and is generally the first echo wave); and (3) the maximum amplitude Amax is substituted into a pre-obtained fitting formula between the maximum amplitude Amax and the solid layer thickness h to obtain the solid layer thickness h of the microcellular foaming injection molding product. The method is advantageous in that the measurement of the solid layer thickness h of the microcellular foaming injection molding product avoids waiting of take-down of the product, and online detection is achieved.