Hey there, let’s work through this issue together—since iOS 11.3.1 doesn’t show up in the standard Xcode component installer, you’ll need to use a few workarounds to get that specific simulator up and running. Here are the most reliable methods:

• First, confirm the version is missing: Open Xcode, go to Xcode > Preferences > Components—you’ll see iOS 11.3 and 11.4 beta, but no 11.3.1 option.
• Head to the Apple Developer Download Center to grab the iOS 11.3.1 Simulator Runtime package (look for the .dmg file labeled specifically for simulators, not device IPSWs).
• Once downloaded, mount the .dmg and locate the .simruntime file inside.
• Copy this file to Xcode’s simulator runtime directory:

  /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes/
  

• Restart Xcode, then go to Window > Devices and Simulators. Click the Simulators tab, hit the + button in the bottom-left corner, and you should now see iOS 11.3.1 in the OS version dropdown. Select your target iPad model and create the simulator.

iOS 11.3.1 is natively supported in Xcode 9.3.2, so this is a more straightforward option if you don’t mind running an older Xcode version alongside your current one:

• Download Xcode 9.3.2 from the Apple Developer Download Center.
• Install it to your Applications folder, and rename it (e.g., Xcode_9.3.2.app) to avoid overwriting your existing Xcode installation.
• Open this older Xcode, go to Xcode > Preferences > Components, and you’ll find iOS 11.3.1 listed under the simulator options. Install it, and you’re ready to test.

If you just need to verify a quick issue that depends on the system version string (not actual runtime behavior), you can trick an existing iOS 11.3 simulator into reporting itself as 11.3.1:

• Open Terminal and run this command to enable debug mode for the simulator:

  defaults write com.apple.CoreSimulator.IndigoFramebufferServices FramebufferDebug -bool YES
  

• Find the UDID of your iOS 11.3 iPad simulator: Go to Window > Devices and Simulators, select the simulator, and copy the UDID from the right-hand panel.
• Run these two commands (replace <SIMULATOR_UDID> with the actual UDID you copied):

  xcrun simctl spawn <SIMULATOR_UDID> defaults write NSGlobalDomain AppleInternal -bool YES
  xcrun simctl spawn <SIMULATOR_UDID> defaults write NSGlobalDomain SystemVersionStampAsString 11.3.1
  

• Restart the simulator. It will now display iOS 11.3.1 in Settings, but note this is only a cosmetic change—under the hood, it’s still running the iOS 11.3 runtime. Use this only for quick checks, not full debugging.

内容的提问来源于stack exchange，提问作者Chris Gao