方法一：
Application.CaptureScreenshot("Screenshot.png", 0); 

方法二：

/// <summary>
/// Captures the screenshot2.
/// </summary>
/// <returns>The screenshot2.</returns>
/// <param name="rect">Rect.截图的区域，左下角为o点</param>
Texture2D CaptureScreenshot2(Rect rect)
{
    // 先创建一个的空纹理，大小可根据实现需要来设置，Scene.width,Scene,height获取全屏
    Texture2D screenShot = new Texture2D((int)rect.width, (int)rect.height, TextureFormat.RGB24,false);

    // 读取屏幕像素信息并存储为纹理数据，
    screenShot.ReadPixels(rect, 0, 0);
    screenShot.Apply();

    // 然后将这些纹理数据，成一个png图片文件
    byte[] bytes = screenShot.EncodeToPNG();
    string filename = Application.dataPath + "/Screenshot.png";
    System.IO.File.WriteAllBytes(filename, bytes);
    UnityEngine.Debug.Log(string.Format("截屏了一张图片: {0}", filename));

    // 最后，我返回这个Texture2d对象，这样我们直接，所这个截图图示在游戏中，当然这个根据自己的需求的。
    return screenShot;
}

方法三：
public void shotPic()
{

    var cam = gameObject.GetComponent<Camera>();

    var renderTexture = new RenderTexture(Screen.width, Screen.height, 24);
    cam.targetTexture = renderTexture;
    cam.Render();

    // 激活这个renderTexture, 并从中中读取像素。
    RenderTexture.active = renderTexture;
    Texture2D screenshot = new Texture2D(Screen.width, Screen.height, TextureFormat.RGB24, false);
    screenshot.ReadPixels(new Rect(0, 0, Screen.width, Screen.height), 0, 0);
    screenshot.Apply();

    // 重置相关参数，以使camera继续在屏幕上显示
    cam.targetTexture = null;
    RenderTexture.active = null;

    //Encode screenshot to PNG
    byte[] bytes = screenshot.EncodeToPNG();
    UnityEngine.Object.Destroy(screenshot);
    File.WriteAllBytes(imagesPath + setPicName("test") + ".png", bytes);

}

u3d摄像机截图