The present invention provides a method for stimulating plant growth, which comprises: (a) placing a light transmissive material for adjusting or retaining light spectrum wavelengths below 500 nm (section A), between 500&tilde630 nm (section B), and above 630 nm (section C) between the light and a photosynthesis receptor of the plant and (b) providing the illuminance or photon flux density of section B lower than that of section A or section C after the light passing through the light transmissive material. The present invention also provides an apparatus and methods for computing cumulative light quantity, comprising (a) a spectrum sensing unit (b) a spectrum multi-band setting module (c) a cumulative light quantity computing module (d) an information processing unit and (e) a control unit.
