  Recent sub-millimeter dust thermal emission observations have unveiled a
significant number of inter-arm massive molecular clouds in M31.However,the
effectiveness of this technique is limited to its sensitivity,making it
challenging to study more distant galaxies.This study introduces an alternative
approach,utilizing optical extinctions derived from space-based telescopes,with
a focus on the forthcoming China Space Station Telescope(CSST).We first
demonstrate the capability of this method by constructing dust extinction maps
for 17 inter-arm massive molecular clouds in M31 using the Panchromatic Hubble
Andromeda Treasury(PHAT) data.Our analysis reveals that inter-arm massive
molecular clouds with an optical extinction(AV) greater than 1.6 mag exhibit a
notable AV excess,facilitating their identification.The majority of these
inter-arm massive molecular clouds show an AV around 1 mag,aligning with
measurements from our JCMT data.Further validation using a mock CSST RGB star
catalog confirms the method's effectiveness.We show that the derived AV values
using CSST z and y photometries align more closely with the input
values.Molecular clouds with AV>1.6 mag can also be identified using the CSST
mock data.We thus claim that future CSST observation could provide an effective
way for the detection of inter-arm massive molecular clouds with significant
optical extinction in nearby galaxies.
