Abstract: Due to the process and thermal fluctuations as well as the reducing of the Tunnel Magneto Resistance (TMR), the read margin of the Spin Transfer Torque Magnetic Random-Access Memory (STT-MRAM) at sub-25 nm process node narrows, which prevents getting desire reading accuracy. To take advantage of the multi-bit soft-decision Error Correction Codes (ECC) coding and decoding algorithm, in this paper, we proposed a two-bit quantizer for the STT-MRAM by using the full-margin Sensitive Amplifier (SA) and Magnetic Tunnel Junction (MTJ) -based threshold. The results show that in the case of small TMR and low read margin, the polar code decoder based on the proposed two-bit quantizer can achieve better output Frame Error Rate (FER) than the Bose, Chaudhuri & Hocquenghem (BCH) scheme based on the one-bit quantizer. We also considered the read decision errors of SA in the STT-MRAM cascaded channel model to accurately evaluate the effect of read margin. The results indicates that the full-margin SA can significantly broaden the effective read margin of the second digit verdict. Moreover, since the induced read decision error decreases, the average output FER becomes 47% lower than that using BCH code with one-bit hard decision quantizer when TMR ≤90%.