The present invention relates to an automatic radioactivity analyzer of mixed liquid beta emitter which comprises: a sample preparation part (1, 2, 3, 4, 5, 6, and 7) for extracting a liquid sample from liquid-phase radioactive nuclear wastes; a sample injection part (9 and 10), including a sample transportation part for transporting a bottled sample to a radioactivity detection part to perform measurement; the radioactivity detection part (11) including two photon multiplier tubes; an exterior gamma-ray source injection part (12) for compensating for measurement efficiency according to quenching effects; a signal processing part (13), including a pre-amplifier circuit (14), a high-voltage generator circuit (15), an analogue-to-digital converter circuit (21), and a digital signal processor(DSP) (24), for generating beta spectrums by the aid of a fast coincidence counter (20) and a multi-channel analyzer (22); a main control PC (25) and a graphic user interface(GUI) program (29) for remotely automatically measuring the sample and analyzing the beta emitter; and an Ethernet communication part (28) for performing the remote-control of the analyzer and long-range reception/transmission of the measured data.