Recently mode-division-multiplexing (MDM) has been widely investigated to enhance fiber optics capacity, in which modes or mode groups in few-mode fiber (FMF) or multi-mode fiber (MMF) are exploited as different spatial channels for data transmission. For short-reach applications, significantly reducing inter-spatial-channel crosstalk to avoid coherent detection and multiple-input-multiple-output (MIMO) equalization is preferred. Currently most studies focus on the design of weakly-coupled FMFs and mode (de)multiplexers. Alternatively, in this work, a wavelength-interleaved (WI) scheme is proposed to mitigate inter-spatial-channel crosstalk by optimizing the design of direct detection (DD) MDM and wavelength-division-multiplexing (WDM) system. In weakly-coupled MDM systems, crosstalk mainly comes from the adjacent spatial channels, and the signal-to-crosstalk beat interference (SCBI) constitutes main crosstalk impairment after square-law detection. The WI scheme interleaves the WDM grids in adjacent spatial channels by half WDM channel spacing and uses an electrical low-pass filtering (ELPF) to remove out-of-band SCBI. The effectiveness of SCBI suppression is theoretically analyzed. The feasibility of WI scheme is experimentally verified by 3-mode 3-wavelength MDM-WDM transmission over 500-m OM3 MMF. Enabled by WI scheme, record 120-km 10G-per-channel MDM-WDM transmission over 2-mode FMF without MIMO equalization is successfully demonstrated. The WI scheme is promising to enhance the performance of short reach or even metro MDM optics.