In this paper, we propose guard-band-shared direct-detection (GBSDD) scheme to simultaneously receive multi-band 100-Gb/s direct-detection optical signal with only one conventional 40-GHz photodiode. The modulation format of orthogonal frequency-division multiplexing based on offset quadrature amplitude modulation (OFDM/OQAM) is selected to provide signal spectrum with high side-lobe suppression ratio, which can effectively reduce the electrical subband frequency interference. In GBSDD scheme, only one guard band is required to accommodate the overlapped signal-to-signal beat interference (SSBI) induced by all the multi-band optical signals. Three different GBSDD schemes are experimentally demonstrated with total data rate of 100-Gb/s and OFDM/OQAM modulation format. The first one uses 6 subbands with transmission distance of 320 km and hybrid 32-QAM and 16-QAM formats. 6 lasers are used as the pilot carriers to beat the signal. The seconds utilizes only 2 subbands with 64-QAM format in 80-km fiber transmission, which are assigned onto two orthogonal polarizations. Only one optical pilot carrier is inserted to beat with the 2 subbands on the two polarizations. The 2 subbands are located on the one side of the optical carrier. Comparing with the first scheme, the bandwidth usage of the PD is enhanced from 1/2 to 2/3. In the third scheme, the 100-Gb/s optical signal includes 4 subbands with 32-QAM format after 880-km fiber transmission. The 4 subbands locate at the two sides of the optical pilot subcarrier. Under this condition, the bandwidth usage of the PD is further improved from 2/3 to 4/5.