The present invention relates to porous silicon preparing apparatus, and is especially one kind of dual bath apparatus for electrochemical process to prepare porous silicon. The dual bath apparatus includes one bath body with fixing slot, one concave plate inserted inside the slot, one base plate and one cushion plate connected with screws to form one concave tank, one clamping plate with several parallel etching windows set in the concave tank, and one other clamping plate with several parallel etching windows possessing notches. The present invention has the advantages of simple porous silicon preparing process, and the prepared porous silicon has homogeneous pore size, homogeneous porosity and layer thickness.