I. applicable to versions before 6.0

You just need to: find the installation path of cloud notes, * \ Youdao\YoudaoNote\theme\build.xml
Open the file with a notebook, find the words "advertisement in the lower left corner", and delete the following code:

<PanelAd type="adpanel" css="public" ass="mainform panelclient PanelAd">
         <panelTopLine type="panel" css="AdPanel" Dockstyle="top" Bounds="0,0,0,1"/>
         <AdPhoto type="photo" css="Ad AdPhoto" ass="common fill"/>
         <AdText type="label" css="AdText" AnchorStyle="topleft" Bounds="20,135,25,10" Margin="0,0,0,0"/>
</PanelAd>

After that, save and restart Youdao cloud notes.

II. 6.0 and later

Find the installation path of Youdao cloud notes, * \ Youdao\YoudaoNote\theme\build.xml
Open the file with a notebook and find the PanelAd tag, which is about 366 lines

<PanelAd type="adpanel" css="public" ass="mainform PanelAd">
    <panelTopLine type="panel" css="AdPanel" Dockstyle="top" Bounds="0,0,0,1"/>
    <MiddlePhotoPanel type="panel" css="public" DockStyle="top" Bounds="0,0,250,160" Margin="0,13,0,13">
        <AdPhoto type="photo" css="Ad AdPhoto" AnchorStyle="center" AutoZoom="ZoomEqual" Bounds="0,0,200,130">
            <AdClose type="button" css="adclear" AnchorStyle="topright" Bounds="-1,1,24,24" />
            <AdText type="label" css="AdText" AnchorStyle="bottomleft" Bounds="6,-6,25,10"/>
        </AdPhoto>
    </MiddlePhotoPanel>
</PanelAd>

Delete the value of panelclient in ass attribute. The screenshot above is deleted.
Then find the label AdWraperMid, which is about 466 lines. Change the values of the bounds property to 0, 0, 0, 0.

<!-- Bottom advertising-->
<AdWraperMid type="panel" css="public" dockstyle="bottom" visible="false" bounds="0,0,0,0">
    <PanelAdMid type="control" control="PanelAd">
    </PanelAdMid>
    <LeftShadow type="control" control="MidPanelLeftShadow"/>
</AdWraperMid>

After that, save and restart Youdao cloud notes.