Most of energy that brain consumes is used to support the intrinsic and spontaneous neural activity.Conditions that alter brain energy metabolism may lead altered intrinsic and spontaneous neural activity.Head-down bed rest,a reliable simulation model for simulated weightless,has proved its role in reducing energy requirements.It is unknown whether intrinsic and spontaneous neural activity measured by resting-state fMRI will be changed after 45 days -6° head-down bed rest.Sixteen healthy volunteers underwent the resting-state fMRI scan before and after 45 days -6° head-down bed rest.Images were acquired with a 3.0 Tesla Siemens MRI scanner.Image preprocessing was performed using SPM8,DPARSF and Matlab.One participant was excluded due to excessive head motion.Amplitude of low-frequency fluctuations (ALFF) was used to measure the local spontaneous activity.After detecting the regions showing changes in ALFF after head-down bed rest,we used functional connectivity analysis based on individual seed region to identify the changes in brain functional network.Paired-sample T test revealed that participants showed decreased ALFF in posterior cingulate cortex and extending to precuneus (PCC/Pcu) after 45 days bed rest (height threshold p<0.01,cluster-level corrected p<0.05).No regions showed significant increased ALFF after 45 days -6° head-down bed rest.Using functional connectivity analysis,we found the decreased positive connectivity between the PCC/Pcu and the medial prefrontal cortex and the decreased negative connectivity between the PCC/Pcu and left supramarginal gyrus after 45 days -6° head-down bed rest (height threshold p<0.01,cluster-level corrected p<0.05).After 45 days -6° head-down bed rest,spontaneous brain activity and functional connectivity in the default mode network was significantly reduced,which may be related to changed energy metabolism caused by long duration bed rest.