Unlike the measurement of the bioelectric signal, the detection of bio-magnetic activities enables noncontact and noninvasive evaluation of electrical activity in living organisms. In this paper, we introduced a feasible method to realize bio-magnetic field detection in non-magnetically shielded environment at the room temperature based on giant magneto-impedance effect (GMI) effect in amorphous wire. Firstly, we review bio-magnetic fields and the principle of GMI effect. Then we carried out off-diagonal GMI effect measurement experiment to obtain the circuit design requirement. Form the experiment result, if the sensitivity of GMI sensors reach to 10 pico-Tesla (pT) level, the gain of amplifier circuit should be more than 10 thousand times. It is almost impossible to obtain so much gain by only one order amplifier circuit, so the circuit was designed using several orders for sake of a stable and high gain (A>=100,000). Customarily in non-magnetically shielded environment, the fluctuation of power frequency interference is about nano-Tesla (nT) level. Hence, some notch filter circuits were designed to eliminate power frequency interference, since doing so can prevent circuit saturation. Considering the bio-magnetic fields distribute in the low frequency domain, so we filtered out the frequency above 200 Hz. In the last part, we carried on performance testing. From the test result, the in-fan base noise of magnification circuit was less than 15mV, and the sensor base noise in the center of 120m2 room without magnetic shielding was below 40mV. We utilized a long enough straight wire to generate weak magnetic field for sensitivity testing, we could infer that the bio-sensor sensitivity is near to 10pT in non-magnetically shielded environment.