Our proposed optical performance monitoring (OPM) technique, based on the multi-task learning, is able to realize modulation format identification (MFI), baud rate identification (BRI), chromatic dispersion identification (CDI), and optical signal-to-noise ratio (OSNR) estimation simultaneously. This OPM technique can be used in the intermediate nodes of optical networks, which is cost-effective since it can monitor systems by direct detection. To further reduce the cost, PD and ADC with low bandwidth are used. Therefore, we investigate the effect of different bandwidths of direct detection receiver on the performance of OPM, to find the optimal low bandwidth of receiver to achieve relatively high OPM performance. The comparison experiment between different bandwidths of receivers has been carried out where signals with two formats, quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (16QAM), two baud rates, 14 GBaud and 28 GBaud, and three CD situations, 0 ps/nm, 858.5 ps/nm, and 1507.9 ps/nm, are adopted. And the test bandwidths of receiver are 0.5 GHz, 1 GHz, 2.5 GHz, 5 GHz, 7.5 GHz, 10 GHz, and the original 33 GHz. It is found that the model with 5 GHz bandwidth receiver has the relatively highest performance, except the model with original 33 GHz bandwidth receiver. The identification accuracies of model with 5 GHz receiver are 99.92%, 99.11%, and 99.94% for MFI, BRI, and CDI, respectively. The OSNR estimation error of this model is 0.594 dB.