The invention relates to a chromium-free magnesium composite brick used for a VOD furnace and a preparation method thereof. The preparation method comprises the following steps: firstly sintering macrocrystalline magnesium sand with yttrium oxide and dicalcium silicate at the temperature of about 1550 DEG C, firstly preparing a pre-prepared material with excellent high temperature mechanical properties by adopting a presintering method, taking the pre-prepared materials in different grain gradations as an aggregate of a mending material, also adopting electrically smelted zirconia and calcined alumina as additives to make an unfired chromium-free magnesium composite brick for replacing an existing fired magnesium chromium brick to be used, pollution of the magnesium chromium brick to the environment is avoided, and thermal shock resistance and high temperature bending strength of a refractory material used for the VOD furnace are improved, so that the service life of the VOD furnace is guaranteed.