The biodegradable AZ31b Magnesium alloy was used in aerospace due to higher strength to low weight ratio, but it is limited in use due to high corrosive material, since by unconventional machining we cannot achieve good geometrical characteristics. So, we use unconventional WEDM, in which spark erosion is used to remove material and achieve a better surface finish. In this work, we perform the experiment on both without heat-treated and heat-treated AZ31b alloys. For heat treatment, we heated the material above recrystallization temperature, i.e., at 500 °C, for 3 h. After that, it cooled in the air at room temperature. After that, the BBD approach of RSM was used to develop the experimental model by considering four input parameters at three levels (Ton = 5–15 µs, Toff = 4–10 µs, I = 1–5 A, WS = 3.12–10.4 m/s for response KW (kerf width), KWD (kerf width deviation), and TA (taper angle), which were investigated by response surface plots. From ANOVA analysis, it reveals that for KW, Ton * I, Ton*WS, and Toff * I, and for KWD, Toff * Ton, Ton * I, and Ton * WS plays a significant role for both materials. For TA Ton * WS for WHT AZ31 alloy, and Ton * WS, Toff * WS plays a significant role for HT3H500 °C AZ31.