Herein, a facile and reliable approach for the determination of silica content in bamboo lignocellulosic waste liquid (black liquor) by inductively coupled plasma atomic emission spectrometry (ICP-AES) in combination with microwave digestion was demonstrated. The results showed that black liquor samples pretreated in a microwave-assisted digestion system with the digestion solution contaning HNO₃, H₂O₂, and HF (volume ratio 10 : 4 : 1) resulted in the conversion of silicides into reactive silica. The resulant samples after the digestion were suitable for the silica analysis by ICP-AES. The ICP-AES results showed that the silica element could be readily determined with high accuracy (recovery was as high as 94.7–102.4%). This approach would enable a rapid analysis of the silica content in a non-wood lignocellulosic waste liquid, thus providing an approach for the comprehensive utilization of the spent liquid resource in a more efficient way.