Strong lens time delays have been widely used in cosmological studies , especially to infer H _ { 0 } .
The upcoming LSST will provide several hundred well measured time delays from the light curves of lensed quasars .
However , due to the inclination of the finite AGN accretion disc and the differential magnification of the coherent temperature fluctuations , the microlensing by the stars can lead to changes in the actual time delay on the light-crossing time scale of the emission region \sim days .
We first study how this would change the uncertainty of H _ { 0 } in the LSST era , assuming the microlensing time delays can be well estimated .
We adopt 1/3 , 1 and 3 days respectively as the typical microlensing time delay uncertainties .
The relative uncertainty of H _ { 0 } will be enlarged to 0.47 \% , 0.51 \% , 0.76 \% , respectively from the one without microlensing impact 0.45 \% .
Then , due to the lack of understandings on the quasar models and microlensing patterns , we also test the reliability of the results if one neglects this effect in the analysis .
The biases of H _ { 0 } will be 0.12 \% , 0.22 \% and 0.70 \% , respectively , suggesting that 1 day is the cut-off for a robust H _ { 0 } estimate .